Preheat oven to 350 degrees and prepare baking sheets with parchment paper.
Add the stick of butter and sugar to a mixing bowl. Cream together until the mixture is smooth.
Mix in the egg and vanilla just until combined.
Then, add the flour just until combined and mix in the cocoa powder, salt, and baking soda.
Stir in 1 cup of mini peanut butter cups.
Add spoonfuls of cookie dough to the prepared baking sheets. Flatten the cookie dough just a bit with your fingers.
Add the remaining chopped peanut butter cups on top and gently press them into the cookie dough.
Bake the cookies for 12-15 minutes, or until the cookies are done in the middle.
Let the cookies sit for a few minutes. Then, transfer to a wire rack to cool completely.
Store cookies in airtight containers or cookie tins.