Begin by lining two baking sheets with parchment paper to prevent sticking. Set them aside while you mix the dough.

In a medium bowl, whisk together 1 ¾ cups flour, ¾ cup cocoa powder, ½ teaspoon salt, and 1 teaspoon baking powder. Set aside.

In a large mixing bowl, cream together ½ cup softened butter, ½ cup light brown sugar, and ½ cup granulated sugar using an electric mixer until light and fluffy.

Add in 2 egg yolks and 1 teaspoon vanilla. Mix until the batter becomes pale and fluffy, about 1 minute.

Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined.

Scoop the dough into 34 equal balls and place them on the prepared baking sheets with space in between.

Refrigerate the shaped dough on the baking sheets for at least 1 hour.

Preheat your oven to 350°F (175°C).

Bake for 9-11 minutes, allowing them to cool on the sheets for 5 minutes before transferring to a wire rack.

In a small saucepan, heat ½ cup heavy cream until it just begins to boil. Pour over 1 cup chocolate chips and stir until smooth.

Once cooled, fill each indentation with about 1 teaspoon of ganache and sprinkle with nonpareils or crushed candy canes if desired.

Allow the ganache to set by chilling the filled cookies in the refrigerator for about 30 minutes before serving.