Line two baking sheets with parchment paper and set them aside.
In a medium bowl, whisk together the flour, cocoa powder, salt, and baking powder. Keep this bowl handy.
In a large bowl, use an electric mixer to cream the softened butter, brown sugar, and granulated white sugar together for about 2 minutes until light and fluffy.
Beat in the egg yolks and vanilla extract to the butter-sugar mix until everything is pale and even fluffier (about 1-2 minutes).
Gently add the dry flour mixture to the wet ingredients. Mix just until everything comes together and no streaks of flour remain.
Scoop out about 1 tablespoon of dough for each cookie (you should get around 34). Roll them into smooth balls and place them on your prepared baking sheets.
Using a 1/4 teaspoon, gently press down into the center of each dough ball to create a little well for the ganache.
Pop the baking sheets with the indented dough into the fridge to chill for at least 1 hour. This helps them keep their shape!
When ready to bake, preheat your oven to 350°F (175°C). Arrange the chilled dough balls about 1 1/2 inches apart on the baking sheets. Bake for 9-11 minutes. (10 minutes is usually just right!).
As soon as the cookies come out of the oven, use the back of a 1/4 teaspoon to gently re-press any indents that might have puffed up. For perfectly round cookies, you can gently swirl a round cookie cutter around each hot cookie. Let them cool on the baking sheet for 5 minutes, then move them to a wire rack to cool completely.
While the cookies cool, put the chocolate chips in a small bowl. Heat the heavy cream in the microwave or on the stove until it's just about to boil. Pour the hot cream over the chocolate chips and let it sit for 1 minute without stirring.
Stir the chocolate and cream until it's completely smooth and glossy. Spoon about 1 teaspoon of ganache into the well of each cooled cookie.
If you like, sprinkle some festive nonpareils on top. Chill the cookies in the fridge for 10-15 minutes to help the ganache set.