Preheat the oven to 350°F, and line 3 baking sheets with parchment paper. Set aside.
Combine the cake mix, butter, and eggs in a large bowl, and mix with an electric mixer until a thick, uniform dough forms. Fold in the Reese's Pieces and peanut butter chips.
Using a 1½ tablespoon cookie scoop, drop level scoops of dough onto the prepared baking sheets, spacing the cookies 2 inches apart. Press a few extra Reese's Pieces and peanut butter chips onto the top of each dough ball.
Bake one sheet at a time for 14 minutes, or until the edges are set. Remove from the oven, and let the cookies cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely. Repeat with the remaining dough to bake the second and third batches.
Store in an airtight container at room temperature.