Halloween Reese’s Monster Cookie Bars are chewy, studded with peanut butter candies, oats, and chocolate chips. Kids and adults will love a monster themed monster cookie bar for their spooky Halloween celebration! Made in one bowl, little prep, and a short bake time – you can have festival treats in no time, sure to impress.
The candy eyes elevate these bars to give them a spooky design, or leave them off to make great bars for any time of the year! I wanted to create a fun peanut butter chocolate dessert that I could also give a Halloween spin. I copied off of my dirt cups and used the Reese’s peanut butter cups to make little “monsters”.
The Reese’s pieces candies add a Halloween touch with the yellow and orange, or use M&Ms. If you like these you’ll enjoy all of my kid-friendly Halloween treats! Find all of my cookie bars recipes here for more inspiration.
What Makes These So Fun
- Festive candy mix-ins: The pops of orange, yellow, and brown from Reese’s Pieces plus the candy eyeballs instantly give these bars Halloween vibes!
- Kid-friendly decorating: Pressing in peanut butter cups and eyeballs is simple enough for kids to help with.
- Customizable: Swap in seasonal candies (M&M’s, sprinkles, or candy corn) to make them work for any holiday.
- Crowd-pleasing flavors: Peanut butter + chocolate is a classic combo everyone loves, so they’re always a hit at parties.
- Crumble topping: crumbling on some of the oat cookie dough gives these a fun layer on top.
Key Bar Ingredients
Unsalted butter: Melted for a rich, chewy base. Salted butter works too, just reduce added salt slightly.
Light brown sugar: Adds sweetness with a hint of molasses; dark brown sugar will give a deeper flavor.
Granulated sugar: Balances the brown sugar for chewy–crisp texture; can swap with coconut sugar for a more caramelized taste.
Eggs: Provide structure and moisture; flax eggs or egg replacer can work for an egg-free version.
Creamy peanut butter: The classic monster cookie flavor; crunchy peanut butter adds texture, or use almond or sunflower seed butter for an allergy-friendly option.
All-purpose flour: Gives the bars structure; a 1:1 gluten-free blend can be substituted.
Old-fashioned oats: Add hearty chew; quick oats will make the bars softer, while skipping oats gives a more brownie-like bar.
Semi-sweet chocolate chips: Melt into the bars for rich flavor; use dark, milk, or even white chocolate chips depending on your taste.
Reese’s Pieces candy: Festive color and peanut butter crunch; swap in M&M’s or seasonal candy for other holidays.
Reese’s Peanut Butter Cups: Chopped for gooey peanut butter pockets; substitute with any favorite chocolate candy bar.
Candy eyeballs: Fun for Halloween; omit or replace with sprinkles for year-round cookie bars.
How To Make The Peanut Butter Bars
Let’s talk a little about how to make these Halloween Reese’s Monster Cookie Bars (The printable instructions are in the recipe card below.)
Quick overview:
- In a mixing bowl combine melted butter, sugars, eggs, and vanilla. Add in peanut butter.
- Fold in dry ingredients. Mix in oats, set 1/4 of dough aside.
- Mix in chocolate and candy, press into pan, crumble extra dough on top. Press in PB cups.
- Bake, add candy eyes, cool, and enjoy!
- Preheat oven to 350 degrees F. Grease or line an 9×13″ baking pan, set aside.
- In a large mixing bowl, combine the melted butter, sugars, vanilla, and eggs. Whisk to combine. Fold in creamy peanut butter until smooth.
- Slowly add the dry ingredients into the wet ingredients and mix until combined. Fold in the oats and set aside 1/4 of the cookie dough mixture.
- To the remaining 3/4 cookie dough, add in the chocolate chips and Reese’s pieces candy (set aside a few extra Reese’s candy to sprinkle on top if desired). Fold gently to combine and add to the prepared baking pan, press down to spread out evenly.
- Take the 1/4 cookie dough set aside and crumble evenly over top. Press in the assorted peanut butter cups (small and large) and sprinkle over any remaining Reese’s pieces.
- Bake at 350 degrees for 35-40 minutes or until a toothpick inserted comes out clean. Remove from oven and let cool on a wire rack, press candy eyes into the warm Reese’s cups. Let cool completely before slicing.
Baking Tips
Use creamy peanut butter for best results.
Bars will be fudgy, allow to cool completely before slicing for best results.
A sprinkle of flaky salt on top adds the perfect touch of sweet and salty.
Line your baking pan with a sheet of parchment paper to easily lift the baked bars out of the pan.
Spray corners of the pan with cooking spray, this ensures your dessert doesn’t get stuck in the pan.
Halloween Reese’s Monster Cookie Bars (One-bowl Dessert)
Equipment
- 9×13" Baking pan
Ingredients
- 1/2 cup Unsalted butter, melted
- 1 cup Light brown sugar, packed
- 1 cup Granulated white sugar
- 2 whole Eggs, room temperature
- 1 tbsp Vanilla extract
- 3/4 cup Creamy peanut butter
- 1 3/4 cups All-purpose flour
- 1 tsp Baking powder
- ½ tsp Salt
- 1 cup Old fashioned oats
- 1 cup Semi sweet chocolate chips
- 9 oz. Reese's Pieces candy
- Assorted Reese's Peanut Butter Cups
- Candy eyeballs
Instructions
- Preheat oven to 350 degrees F. Grease or line an 9×13" baking pan, set aside.
- In a large mixing bowl, combine the melted butter, sugars, vanilla, and eggs. Whisk to combine. Fold in creamy peanut butter until smooth.1/2 cup Unsalted butter, 1 cup Light brown sugar, 1 cup Granulated white sugar , 2 whole Eggs, 1 tbsp Vanilla extract, 3/4 cup Creamy peanut butter
- Slowly add the dry ingredients into the wet ingredients and mix until combined. Fold in the oats and set aside 1/4 of the cookie dough mixture.1 3/4 cups All-purpose flour, 1 tsp Baking powder, ½ tsp Salt, 1 cup Old fashioned oats
- To the remaining 3/4 cookie dough, add in the chocolate chips and Reese's pieces candy (set aside a few extra Reese's candy to sprinkle on top if desired). Fold gently to combine and add to the prepared baking pan, press down to spread out evenly.1 cup Semi sweet chocolate chips, 9 oz. Reese's Pieces candy
- Take the 1/4 cookie dough set aside and crumble evenly over top. Press in the assorted peanut butter cups (small and large) and sprinkle over any remaining Reese's pieces.Assorted Reese's Peanut Butter Cups
- Bake at 350 degrees for 35-40 minutes or until a toothpick inserted comes out clean. Remove from oven and let cool on a wire rack, press candy eyes into the warm Reese's cups. Let cool completely before slicing.Candy eyeballs
Notes
- Candy eyes can be left off to make these a good dessert for year round!
- Customize the candy: Swap in candy corn, Halloween M&M’s, or chopped peanut butter cups for a fun variety.
- Don’t overbake: The bars should look slightly underdone in the center when you pull them out, they’ll firm up as they cool.
- Perfect slices: Let the bars cool completely before cutting to get clean edges and gooey centers.
- Make ahead: These store well for 3-4 days in an airtight container and can be frozen for up to 2 months.
- Cut into strips, squares, or triangles to get the most out of these for serving Halloween parties.
Nutrition
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Storage & Reheating
- Room temperature: Store bars in an airtight container at room temp for up to 4 days.
- Freezer-friendly: Wrap cooled bars tightly in plastic wrap, then place in a freezer bag or container for up to 2 months. Thaw at room temperature before serving.
- Reheating: These bars are best enjoyed at room temp, but if you love a warm, gooey bite, pop a square in the microwave for 8–10 seconds before serving.
Recipe FAQs
Room temperature eggs mix together a lot easier than cold eggs, resulting in lighter, fluffier baked goods! Room temperature eggs also whip up a lot better which makes them important to use when making meringue or items that call for a lot of eggs like, angel food cake. Don’t have room temperature eggs? Set a few cold eggs out 20-30 minutes before you need them.
Yes! Combine in a stand mixer fitted with a paddle attachment or with a electric hand mixer.
No, the dough for these cookie bars does not need any time to chill – simply make and bake! If you need to, chill the dough for 1-2 hours or overnight.
Yes, if you desire a thicker, more dense bar, these can be made in an 8? x 8? pan. The baking time will need to be increased to 45-50 minutes so the dough cooks through in the middle. Note, this may cause the edges of the bars to get a bite crispier due to the longer baking time.
The dough for cookie bars is the same as individual cookies, the difference is that cookie bars or blondies are spread out into a baking pan instead of scooped or dropped onto a baking sheet. Typically, dough for cookie bars is thick just like dough for individual cookies.
Yes! Quick oats will give the bars a softer texture, while old-fashioned oats add a bit more chew. Both work well.
No! You can swap in Halloween M&M’s, candy corn, or even chopped peanut butter cups for a different twist.
Tools I Recommend
- USA Pan 9×13 – my go-to 9×13 and 8×8 pans, I love the sharp square corners and my desserts release easily!
- Rubber Spatula – a must have! I like these because they don’t come apart so I don’t have to worry about water getting inside the spatula and causing mold.
- Pyrex Mixing Bowl Set – I love this set of nesting bowls. Pyrex glass does not break easily so you can trust on these bowls for years, they come in 3 useful sizes and can stack.
- Parchment Paper – love this for lining pans, it helps to easily life out bars and brownies for slicing.
Did you try this recipe?
Let me know what you thought by leaving a comment below and sharing your final dish on Instagram, Facebook, & Pinterest!
Cheers!
-Rachel
