1cupconfectioners' sugar (plus more if needed), sifted
3tablespoonsunsweetened cocoa powder
1/4cupunsalted butter, VERY soft
Chocolate sprinkles, optional
Preheat oven to 350 degrees (F). Spray a 9-inch tart pan (link in post to the one I use) lightly with nonstick spray; set aside.
In a large bowl using a hand-held mixer - or in the body of a stand mixer fitted with the paddle attachment - beat the butter and peanut butter on medium speed until completely smooth. Add in the brown sugar and continue to beat on medium speed until creamy; about 1 minute. Add in the egg and vanilla, increase speed to medium-high, and beat until combined, scraping down the sides and bottom of the bowl as needed; set aside.
In a separate, medium-sized bowl, combine flour, baking powder, and salt. Turn the mixer on to low speed and carefully add the dry ingredients to the wet ingredients, beating just until combined. Using a rubber spatula stir in the reese's pieces; stir until just combined.
Use a sturdy spatula to spread the cookie dough (it will be thick!) into the prepared pan; press evenly into pan. Bake for 11-14 minutes, or until the center is set and the edges are golden. Once removed from the oven, allow cookie cake to cool completely. Once cooled, remove the cookie cake from the tart pan OR you can serve the cookie cake directly from the pan.
For the chocolate frosting:
Sift together the confectioners' sugar and cocoa powder, pressing it through a small sieve and into a large bowl to assure there are no lumps left; set aside.
Beat the butter on medium speed until smooth and creamy; about 2 minutes.
Gradually add the sifted sugar/cocoa powder alternately with the milk. Stir in vanilla and salt.
Once all of the ingredients have been added, beat on high speed until creamy and combined; about 2 minutes.
Decorate the cooled cookie cake with frosting and chocolate sprinkles. Cut into pieces and serve at once!