In a medium sized bowl, combine the four, cocoa powder, baking soda and salt. Set aside.
In a large bowl of a stand mixer fitted with a paddle attachment (or just a large bowl with a hand mixer,) cream the butter and sugars until smooth and fluffy. Add the egg and vanilla and mix until well-combined.
Gradually add in the dry ingredients and mix until incorporated with no flour-y streaks (but don't over-mix.) Fold in about half of the peanut butter cups.
Chill dough in the fridge for 45 minutes or in the freezer for 15. Preheat oven to 350F.
Scoop dough into 1 1/2 inch rounds and place onto a parchemnt-lined cookie sheet 2/3 inches apart. Press a few peanut butter cup bits into the top of each dough ball. Bake for 10-12 minutes or until the edges are just set.