Preheat the oven to 350°.
Cream together butter and sugar in a stand mixer or electric hand mixer. Add eggs one at a time.
Add vanilla and cocoa. Mix together until combined.
Add flour one cup at a time and mix until combined. Stir in the peanut butter cup pieces with a wooden spoon.
Pour batter in a greased 9x13 pan and bake 30-35 minutes.