Line a cookie sheet with parchment paper, set aside. In a small mixing bowl combine the flour, cornstarch, baking soda, and salt, stir to combine.
In the bowl of a hand or stand mixer fitted with a paddle attachment cream together butter and sugars until fluffy, 3-4 minutes. Add in the peanut butter and mix to combine. Add in the eggs, one at a time, and the vanilla. Scrape down the sides of the mixer bowl. Slowly add in the dry ingredients until fully incorporated.
Gently mix in the oats, chopped peanuts, chocolate chips, and M&M's. Cover and chill dough for 30 minutes. After dough has chilled preheat oven to 350 degrees F.
After chilling, use a cookie scoop to scoop even sized chunks of cookie dough. Using a measuring spoon or a cookie scoop, scoop out even amounts of dough, about 1 1/2 tablespoons. Roll into small balls until round. If desired press reserved chocolate chunks on top of unbaked cookies. Repeat until all of the dough has been used. Arrange 2 inches apart on the prepared cookie sheet.
Bake in a preheated 350 degree oven for 12-14 minutes until golden brown. Allow to cool slightly, transfer to cooling rack and allow to cool completely. These will keep in an airtight container for up to a week!
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