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

In a medium bowl, whisk together flour, cocoa powder, salt, and baking powder. Set aside.

In a large bowl, cream together butter, light brown sugar, and granulated sugar until light and fluffy.

Mix in egg yolks and vanilla extract until fully incorporated.

Gradually add the dry ingredient mixture to the wet mixture until just combined.

Form the dough into approximately 34 balls, place on prepared sheets, and create an indentation in the center of each.

Refrigerate the cookie dough balls for at least 1 hour to maintain shape during baking.

Bake cookies for 9-11 minutes until edges are set but center is still soft.

Prepare ganache by pouring hot cream over chocolate chips; stir until smooth.

Fill each cookie indentation with ganache and optionally sprinkle with nonpareils.

Store cookies in an airtight container for up to 3 days at room temperature.