Preheat oven to 350° F. Line a baking sheet with parchment paper.
In a medium bowl, wisk together flour, cocoa, baking soda, and salt.
In the bowl of an electric mixer, cream the butter, peanut butter, and sugars until light and fluffy (about 3 minutes).
Beat in egg and vanilla until smooth
Gradually add dry ingredients and mix just until combined.
Gently fold in 1 ½ cups chopped peanut butter cups.
Using large (2 tablespoon) scoop, place dough on prepared baking sheet 2”-3” apart. For flatter cookies press down slightly with finger tips. (Thicker cookies are better for mailing as they stay soft longer.)
Gently press 3 or pieces of chopped peanut butter cup onto the top of each cookie.
Bake 10-12 minutes until tops of cookies look set. (Cookies will continue to firm up out of the oven. Over baking will give you a dry cookie.)
Cool for 10 minutes on the baking sheet before transferring to a wire rack to cook completely.
If wrapping in plastic wrap for mailing, place the cookies in the refrigerator or freezer for about 10 minutes to harden the peanut butter cup pieces on top of the cookie.
Store in an airtight container at room temperature for up to 5 days. Cookies wrapped in plastic then stored in an airtight container stay fresh up to 7 days.