Cream together butter, peanut butter, sugar, and brown sugar. Beat until light and fluffy, about 5 minutes, scraping the bowl a few times.
Add the vanilla and egg.
Add the flour, cocoa, baking soda, and salt.
Mix until the dough comes together.
Add the Reese’s Pieces and blend until they are incorporated. Do not mix too much, or the Reese’s will break.
Drop onto parchment lined cookie sheets and bake for 8 to 12 minutes in a preheated 350 degree oven. The edges will be set, and the centers will still be soft.