Make the Cookie Dough:

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 mixing bowl, cream the butter, brown sugar, and granulated sugar together for 2 minutes.

Add egg yolks and vanilla, mixing for another 1–2 minutes until pale and fluffy.

Add dry ingredients and mix just until combined.

Scoop dough into 34 1-tablespoon portions. Roll into balls and place on prepared baking sheets.

Press a 1/4 tsp measurement into each ball to create a thumbprint indent.

Chill the indented dough for at least 1 hour.

Bake the Cookies:

Preheat oven to 350°F (175°C).

Arrange chilled cookie dough 1.5 inches apart on baking sheets.

Bake for 9–11 minutes (10 minutes is ideal).

Re-press centers gently with a 1/4 tsp if needed. Optionally use a round cutter to reshape cookies while warm.

Let cool on the baking sheet for 5 minutes, then transfer to wire racks.

Make the Ganache:

Place chocolate chips in a heat-safe bowl.

Heat heavy cream until just boiling, then pour over chocolate. Let sit 1 minute.

Stir until smooth and glossy.

Assemble:

Spoon ~1 teaspoon of ganache into the center of each cooled cookie.

Garnish with sprinkles if desired.

Chill for 10–15 minutes to set the ganache.

Store in an airtight container for up to 3 days.