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.
1 1/2 cup all-purpose flour, 6 tablespoon Dutch-process cocoa powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt
In a large mixing bowl, blend together butter, peanut butter, and sugars until creamy.
6 tablespoon unsalted butter, 1/4 cup plus 2 tbsp. creamy peanut butter, 1/2 cup granulated sugar, 1/2 cup brown sugar
Add in the egg, vanilla extract, and milk and blend until smooth.
1 large egg, 1 teaspoon vanilla extract, 2 tablespoon milk
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.
1 ½ cups of coarsely chopped Reese's 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.
½ cups of coarsely chopped Reese's peanut butter cups*
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.