Preheat oven to 350°F. Line 2 baking sheets with parchment paper or a baking mat. Set aside.
In a stand mixer or large bowl, combine the peanut butter, softened butter, brown sugar, and granulated sugar. Cream together for 1-2 minutes, until the mixture is fluffy. Add in the egg and vanilla extract and mix until combined.
Add in the flour, cocoa powder, baking soda, salt, and espresso powder and mix to combine. Fold in the Reese's Pieces.
Using a medium cookie scoop, scoop out cookie dough and place cookies about a few inches apart on the prepared baking sheet.* Top with more Reese's candies, if desired.
Bake cookies for 13-15 minutes. Edges will be firm and the center will be slightly soft. Remove from oven and let rest on the baking sheet for a few minutes. Transfer to a cooling rack.
Notes
*You should be able to fit 6-8 cookies per baking sheet.