Line two baking sheets with parchment paper.

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

In a large bowl, cream the butter, brown sugar, and granulated sugar for 2 minutes.

Add egg yolks and vanilla; mix until pale and fluffy, 1–2 minutes.

Add dry ingredients and mix until just combined.

Scoop dough into 34 portions (about 1 tbsp each), roll into balls, and place on baking sheets.

Press the center of each ball with a ¼ teaspoon to create an indent.

Chill for at least 1 hour.

Preheat oven to 350°F.

Arrange dough balls 1½ inches apart and bake for 9–11 minutes.

Re-indent the centers while still warm; use a round cutter to shape if desired.

Cool on the pan for 5 minutes, then move to a rack.

For the ganache: place chocolate chips in a bowl. Heat the cream until nearly boiling, pour over the chocolate, let sit for 1 minute, then stir smooth.

Spoon about 1 tsp of ganache into each cookie.

Add sprinkles if desired.

Chill 10–15 minutes to set.

Store in an airtight container for up to 3 days.