Preheat the oven to 350 F. In a large bowl, mix together butter, peanut butter, and powdered sugar until smooth. Add in the vanilla extract and flour until it is fully combined.
Fold in the Reese's pieces. Roll the cookie dough into balls and flatten each one slightly. Place the cookies onto a large ungreased baking sheet at least 2-inches apart. Bake for 10-12 minutes.