Preheat the oven to 350 F. In a large bowl, combine butter, peanut butter, and powdered sugar and mix until fluffy. Add in the vanilla extract and mix until smooth.
Add in the flour and mix until it is fully combined. Fold in the mini Reese's peanut butter cups.
Roll the cookie dough into balls, and flatten them slightly. Place the cookies onto a large ungreased baking sheet. Bake for 10-12 minutes.