Line 2 baking sheets with parchment paper or silicone baking mats. In the bowl of an electric mixer, combine the butter and sugars. Beat together on medium-high speed until light and fluffy, 2-3 minutes. Blend in the egg and vanilla, scrape down the bowl if needed.

In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt. Slowly add the dry ingredients into the wet ingredients in the mixer on low speed just until incorporated. Mix in the milk on the lowest speed and mix until incorporated. Add the chopped peanut butter cups and gently fold in by hand. Preheat the oven to 350˚ F.

Roll about 2 tablespoons of dough into balls and place on the baking sheets. Top with some more chopped up peanut butter cups if desired. Place cookie sheet in freezer for 10-15 minutes. (Alternately you can chill the dough for at least 2 hours or up to 36.)

Bake 10-11 minutes. Let cool on the baking sheets for about 5 minutes, then transfer to a wire rack to cool completely.