Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a mixing bowl, whisk together flour, cocoa powder, salt, and baking powder until combined. Set aside.

In a large bowl, cream together butter, light brown sugar, and granulated sugar for about 2 minutes. Add egg yolks and vanilla, mixing until creamy.

Gradually add the dry mixture to the wet mixture, stirring until just combined to create the dough.

Portion out the dough and roll into balls, creating an indent in the center of each ball. Chill for at least 1 hour.

Bake for 9 to 11 minutes, ensuring they remain soft in the center. Re-indent if necessary after baking.

Heat the whipping cream until boiling, then pour over chocolate chips in a bowl and stir until smooth to make ganache.

Fill each cookie indent with ganache and optionally sprinkle nonpareils on top. Refrigerate for 10 to 15 minutes.