Start off by making the filling. Grab a bowl and combine the peanut butter and powdered sugar and mix together until well combined.
Taking 1-2 tsp sizes, roll the filling into balls using your hands (your hands are the best tool in the kitchen;) The warmth from your hands will help form the balls easy!
Set aside and start on the cookie dough.
In a stand mixer, cream together butter and sugars. Ideal creaming time is 3-5 minutes on medium speed, scraping the bowl every now and then with a spatula.
Add in egg and vanilla and continue mixing.
Take the flour, cocoa powder, and baking soda, and add into the mixing bowl. Mix on a low speed until a thick dough forms.
Scoop out about 1 tbsp of dough onto your prepared baking sheet. Press it down firmly and add one of the peanut butter balls that was made previously on top of each cookie. Top off with additional cookie dough and 'close' the cookie (See step-by-step photos above).
Place into refrigerator to chill for 30 minutes. Once chilled, bake at 350°F for 12-14 minutes. Top off with dark chocolate chips if wanted.
Enjoy!