Preheat the oven to 350 degrees and line a baking sheet with parchment paper.

In a large bowl, cream together butter and peanut butter until well incorporated. Add in brown sugar and white sugar; cream. Beat in the egg, scraping down the sides of the bowl as necessary. Lastly, add in vanilla extract and salt. Set aside.

In a separate bowl, sift together flour, cocoa powder, baking soda, cornstarch, and espresso powder. Slowly incorporate the dry ingredients into the wet ingredients. Dough will be thick.

Once combined, add in milk, beginning with 2 Tbsp. If the dough still seems extra thick, add in the last Tbsp.

Add in 1/2 cup semi-sweet chocolate chips and 1/2 cup Reese's cups (save the 1/4 cup for later); mix.

With a medium cookie scoop (about 2 Tbsp of dough), place the dough evenly on a cookie sheet, about 6 per tray.*

With the remaining Reese's cups, place 1-2 pieces in each ball of cookie dough.

Bake for 14-16 minutes, or until the tops of the cookies look firm.

Enjoy with milk!