Preheat the oven to 350°F. In a stainless steel mixing bowl, whisk together flour, baking powder, cinnamon, salt, and nutmeg. Set aside.
Using a stand or hand mixer fitted with a whisk attachment, whip the egg whites on high speed. While whipping, slowly sprinkle in the sugar. Continue to whip until all of the sugar has been incorporated and the whites are glossy and frothy.
Turn down to the lowest speed and add in egg yolks. Don’t skip this step! Adding the egg yolks and egg whites separately is the secret to the spongy texture.
Slowly add in the dry ingredients until they’ve all been incorporated. Pour the batter into a shallow 3-quart baking dish. Bake for 25-30 minutes or until a toothpick comes out clean when inserted into the middle of the cake. Remove the cake and allow to cool for 15 minutes.
While the cake is cooling, whisk together the eggnog and whiskey. Set aside so that the flavor can marinade, but do not put back in the fridge.
After the cake has cooled for 10 minutes, poke holes in the top of the cake using a wooden skewer or a fork. Pour half of the eggnog mixture onto the cake, then let the mixture sink into the pores of the cake for about 15 minutes, then add the rest of the eggnog.
Wrap the cake in plastic wrap and let sit in the fridge for several hours or overnight, whichever is preferred. When you’re ready to serve the cake, whip the quark with a fork in a small bowl to get the texture evenly smooth.
Whisk in the confectioner’s sugar and vanilla extract. Spread the quark mixture over top of the soaked cake and grate nutmeg over the top. Serve immediately! (Why wouldn’t you?)