Preheat the oven to 350 F. Line a 12 cup muffin pan with paper liners. In a large bowl, mix together cake mix, eggs, vegetable oil, and peanut butter until smooth.
Evenly divide the cookie dough between the prepared muffin pan. Bake for 12-15 minutes. Remove from the oven, and place one Reese's peanut butter cup on each cookie cup.