Measure into a medium bowl, the gluten free 1 to 1 flour or substitution flours, cornstarch, baking soda, and salt (if using). Stir together.
Melt the chocolate in the microwave for 15-25 seconds.
Cream the butter and the sugars together in the bowl of a stand mixer or in a bowl with a hand mixer. Add the eggs and mix in. Add the melted chocolate and mix on medium speed. Next add the cocoa powder mixing on low speed. Mix in the vanilla.
Add the flour mixture and mix in until just combined. Stir in by hand the chocolate chips and the peanut butter cup pieces, reserving ⅛ of the amount of peanut butter cup pieces for the cookie tops.
Chill the dough for 1-2 hours.
Preheat the oven to 350° F. Scoop or form 1 ½ inch cookie balls and place six at a time on a baking sheet lined with parchment. (Or make 1” cookie balls and bake for less time). Press one or two peanut butter cup pieces to the top of each cookie ball.
Bake each tray for 11 -14 minutes. Let cookies cool on the baking sheet for 3-5 minutes before carefully removing to a parchment lined wire rack to finish cooling. Best if eaten within 3 days.