Preheat your oven to 350 degrees. Grease both 9-inch cake pans with butter, dust them with flour, and set them aside.
In a stand mixer fitted with a paddle attachment, beat the butter, sugar, and brown sugar until the batter is fluffy and light in appearance, about 2 minutes.
Add the vanilla extract and sour cream and beat until combined.
Add 2 egg whites at a time, stopping to scrape down the sides of the bowl with a silicone spatula. When all of the egg whites have been blended into the batter, add the whole egg. Beat until the ingredients are thoroughly combined.
Add 1 1/4 cup flour, baking powder, and nutmeg. Mix until combined.
Pour 1 cup of eggnog into the batter. Blend well. Scrape down the sides of the bowl, and then add the remaining flour and spiced rum. Blend the mixture until it is smooth.
Divide the batter between the 9-inch cake pans. Lightly tap the pans on the counter before placing them into the oven to dislodge any large air bubbles. Bake for 21-25 minutes, or until a toothpick inserted in the center comes out clean.
Let the cakes cool in their pans for 10 minutes. Run a knife around the edges of the cakes to loosen them from the sides of the pans. Then invert the cakes onto the cooling racks and allow them to cool entirely.
While the cake cools, prepare the frosting for the crumb coat. In a stand mixer fitted with a whisk attachment, blend the butter with 2 cups of powdered sugar and the nutmeg, salt, and eggnog.
Scrape the sides of the bowl with a silicone spatula. Add the remaining cup of powdered sugar and whip until the frosting is smooth. If the frosting is too thick, add a teaspoon of eggnog at a time and whip until you reach the desired consistency. If the frosting is too thin, beat in a small amount of powdered sugar.
When your cakes are completely cool, use a serrated knife to level off your cakes carefully.
Place the first layer of cake on a cake stand and add the buttercream to the center. Spread an even thick layer of buttercream using a long offset spatula. Take little bits of frosting from the center and lightly sweep them over the edges of the cake. Spread a thin layer on the sides, just enough to lock in the crumbs of the cake.
Place the top layer of the cake onto the bottom layer. Repeat the same steps you used to spread buttercream over the bottom layer, until the entire cake is covered in a thin layer of buttercream.
Place the cake in the refrigerator to chill for at least 30 minutes to allow the buttercream to firm up for the final coat of frosting.
Prepare the frosting for the final coat. In a stand mixer fitted with a whisk attachment, blend the butter with 2 cups of powdered sugar and the nutmeg, salt, and eggnog.
Scrape the sides of the bowl with a silicone spatula. Add the remaining cup of powdered sugar and whip until the frosting is smooth.
Set aside 1 1/2 cups of frosting for decorating. Using a long offset spatula, top the center of your cake with eggnog buttercream, spreading the frosting evenly just past the edges of the top surface. Use the overhang of buttercream to coat the sides of the cake.
Place the 1 1/2 cups of decorating frosting you set aside into a disposable piping bag fitted with a Wilton 1M piping tip. On the top of the cake, create a border of ruffle/beehive mounds. On the bottom layer, using a disposable piping bag fitted with a Wilton 8 piping tip, create a trim of pearls. Garnish the cake with festive sprinkles.
Store the cake in the refrigerator until you are ready to serve it. Allow the cake to come to room temperature before serving, about 20 minutes. Enjoy!
Substitute a teaspoon of rum extract for 2 tablespoons of spiced rum.