Preheat oven to 350 °F (177 °C). Line a large baking sheet with parchment paper or spray with baking spray. Set aside.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.
In a large mixing bowl, blend together butter, peanut butter, and sugars until creamy.
Add in the egg, vanilla extract, and milk and blend until smooth.
Slowly add in the dry ingredients and blend until well-incorporated. The dough will be fairly thick.
Gently stir in 1 1/2 cups of the peanut butter cups.
Drop 2 tbsp. rounds of dough onto prepared cookie sheet, spacing them 2-3 inches apart.
Take your leftover peanut butter cups and gently press a few on top of the cookie balls.
Bake for 12-14 minutes.
Remove from oven and let sit on baking sheet for about 5 minutes to firm up. Transfer them to a wire cooling rack to cool completely.
Enjoy with a large glass of milk :)
Notes
I usually put my Reese's in the fridge or freezer upon bringing them home because I like to eat them cold or frozen. This came in handy because they're much easier to cut when they're cold than when they're room temperature.