In a stand mixer, combine butter, peanut butter, brown sugar and white sugar and beat until creamy. Add egg and vanilla. Beat until well combined.
Sift together flour, cocoa powder, baking soda and salt. Pour into mixer and beat on medium-low speed until dry mixture is incorporated into wet ingredients.
Add Reese's Pieces on low speed just until mixed into dough.
Scoop dough by spoonfuls and roll into balls, keeping sizes the same. Place on ungreased baking sheet, leaving room between balls for expansion.
Using the back of a mug or glass, press into each ball to form circular disk. Wet the back of the glass with cold water if it's sticking.
Press Reese's Pieces into cookie dough in pattern of your choice.
Bake in upper half of oven for 10 minutes. Cool on a rack.
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