Preheat oven to 350 degrees F, line baking sheets with parchment paper or silicone mats, set aside.
In a mixing bowl combine the sugar cookie mix, melted butter, eggs, and extra flour. Mix until a shaggy dough forms. The dough will be sticky.
Transfer the dough onto a floured surface and gather into a ball. Roll the dough out until it is about an 1/2" thick*. Use cookie cutters to cut the dough into desired shapes. Transfer to the prepared baking sheets, repeat with remaining dough.
Bake at 350 for 10-12 minutes until slightly golden brown the cookies will still be soft. Allow to cool on the baking sheet before transferring to a wire rack. Cool completely before frosting.
For the Frosting:
In the bowl of a stand mixer or if using a hand mixer, a large mixing bowl, cream the softened butter until light and fluffy, about 2-3 minutes. Add in the vanilla extract and heavy cream, mix to combine.
With the mixer on low add in 1/2 a cup of powdered sugar at a time, mixing until combined each time. Scrape down the sides occasionally. Continue to add powdered sugar until you have reached a desired consistency, you may not need all 4 cups.*
Spread or pipe the frosting onto the cooled cookies, decorate with colored sugar or sprinkles as desired!
Notes
*You can work dough in batches if it is easier. I prefer keeping the thickness at 1/2", but you can make them thinner or thicker, just adjust baking time. *I like to keep the buttercream thick so it holds up on the cookie, thin buttercream if desired by adding more heavy cream.