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Preheat the oven to 350 degrees and line a baking sheet with parchment paper.
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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.
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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.
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Once combined, add in milk, beginning with 2 Tbsp. If the dough still seems extra thick, add in the last Tbsp.
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Add in 1/2 cup semi-sweet chocolate chips and 1/2 cup Reese's cups (save the 1/4 cup for later); mix.
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With a medium cookie scoop (about 2 Tbsp of dough), place the dough evenly on a cookie sheet, about 6 per tray.*
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With the remaining Reese's cups, place 1-2 pieces in each ball of cookie dough.
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Bake for 14-16 minutes, or until the tops of the cookies look firm.
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Enjoy with milk!