Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper. Set aside.
In a large mixing bowl, beat together the cake mix, melted butter, and eggs with a hand mixer until combined. Pour in the Reese's pieces and stir in by hand with a rubber spatula.
Using a medium cookie scoop, scoop the cookie dough into 2-tablespoon portions. Place the dough balls on the prepared baking sheet 3 inches apart. Top each dough ball with extra Reese's Pieces if desired.
Bake for 8-10 minutes or until the cookies are set in the center.
Remove the cookies from the oven and allow to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.