How to Make It
Preheat the oven to 350° and line 2 large baking sheets with silicone mats or parchment paper. In a small bowl, whisk the flour with the cocoa powder, sugar and salt.
In the bowl of a standing mixer fitted with the paddle, beat the butter at medium speed until creamy. Beat in the egg yolk. Add the dry ingredients and beat at low speed until incorporated. Pat the dough into a disk, wrap in plastic and refrigerate until chilled, about 30 minutes.
On a lightly floured surface, roll out the dough a scant 1/4 inch thick. Using a 2 1/4-inch round cookie cutter, stamp out as many rounds as possible. Reroll the scraps and stamp out more rounds. You should have about 40 rounds. Set a peanut butter cup or halved mounds bar in the center of each round and roll the dough around the candy, pinching any tears. Roll the cookies into balls and arrange on the prepared baking sheets, 1 inch apart.
Bake in the cookies on the upper and middle racks of the oven for about 16 minutes, until the dough is firm and just slightly browned; shift the pans from top to bottom and front to back halfway through baking. Let the cookies cool completely on the baking sheets.
In a microwave-safe bowl, melt the bittersweet chocolate on high power in 30-second bursts until nearly melted. Remove from the microwave and stir until melted. Repeat with the white chocolate in another microwave-safe bowl. Dip the tops of the cookies in the melted chocolate and decorate with sprinkles, dragees and nonpareils. Transfer the cookies to a parchment paper—lined baking sheet and refrigerate until the chocolate is set, about 10 minutes. Transfer to a platter and serve.