Line baking sheets with parchment paper or silicone mats.

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

In a large mixing bowl, beat the butter and brown sugar on medium speed until light and fluffy, about 1–2 minutes.

Beat in the egg and egg yolk for 30 seconds, scraping down the sides as needed.

Gradually add the dry mixture to the wet ingredients, mixing on low speed until just combined. The dough will be very thick.

Scoop 1 tablespoon of dough (use a #60 cookie scoop) and roll into smooth balls. Roll each ball in granulated sugar and place 2 inches apart on prepared baking sheets.

Use the back of a teaspoon to press a divot into the center of each cookie before baking. Place the baking sheet in the freezer for 10 minutes to firm up the dough. You can freeze longer if needed.

Preheat the oven to 350°F (177°C). Bake for 10-11 minutes or until the edges are set but the centers still look soft. Remove from the oven and immediately use the teaspoon again to gently deepen the center indentation as it disappears while baking.

Allow cookies to cool completely before filling.

Place the white chocolate chips in a bowl. In a microwave-safe measuring cup, microwave the eggnog for 30 to 45 seconds, or until steaming and very hot. Pour the hot eggnog over the white chocolate chips and let it sit for 2 to 3 minutes. Stir until the chocolate is fully melted and the ganache is smooth. If needed, microwave the mixture in 10-second bursts to finish melting. Allow the ganache to set and thicken for at least 30 minutes. Speed up the process by putting it in the fridge.

Once thickened, spoon about 1 teaspoon of the eggnog ganache into the center of each cookie. Lightly dust the tops with ground nutmeg. Allow the ganache to set before serving or storing.