Preheat the oven to 350 degrees F. Place an oven rack in the middle position. Lightly grease the bottom of 3 (8-inch) cake pans (I use baking spray with flour) and cover them with parchment rounds. Set aside.
In a medium bowl, whisk together the flour, cornstarch, baking powder, salt and cinnamon. In a liquid measuring cup, whisk together the eggnog, rum and vanilla extract.
In a stand mixer with the paddle attachment, cream the butter and sugar together on medium-high speed for 5 minutes until light and fluffy. On low speed, add the eggs one at a time, periodically scraping down the bowl and allowing each egg to incorporate before adding the next. Scrape down the sides of the bowl again. With the mixer medium-low speed, quickly alternate between adding the dry and liquid ingredients (ending with the dry) until just combined. Scrape down the sides well, making sure to reach the bottom of the bowl. Turn the mixer up to medium-high and allow the batter to mix for an additional 30 seconds.
Distribute the batter evenly between the 3 prepared cake pans (for best results use a kitchen scale to weigh out each pan). Use a spatula to level out the tops of the cakes as much as possible.
Bake until the a toothpick comes out of the center of each cake clean, approximately 33-36 minutes. Allow the cakes to cool completely in the pan before removing. Allow the cake to cool completely (make ahead: prepare the cakes the day before and allow to cool overnight).
Prepare the buttercream:
Add sugar and water to a medium sauce pan. Cover and turn the heat to medium-high. Once the liquid begins to simmer and steam has developed, remove the cover (the steam helps prevent sugar crystallization). Using a candy thermometer, cook the sugar to the soft boil stage, 235-240 degrees F, approximately 5 minutes.
While the sugar is cooking, Place the egg whites in a stand mixer with the whisk attachment. Whisk on high speed until a soft peak has formed.
Turn the mixer down to medium-low and slowly pour the syrup down the side of the bowl into the egg whites. Don’t pour the hot syrup directly into the meringue or the eggs might scramble.
Once the syrup is completely incorporated, turn the speed to high and allow the meringue to continue forming a stiff peak while cooling to room temperature. To speed up this 15-20 minute process, ice packs can be placed around the bowl.
Once the meringue is at room temperature, very slowly begin incorporating the butter on medium speed. Once all of the butter has been incorporated, turn the mixer up to high speed and slowly add the eggnog, vanilla extract, rum and nutmeg.
Assemble the layer:
Place the first layer of cake on a revolving cake stand and remove the parchment paper. Placing a cardboard round below the cake is optional but will make Top the cake with 1 cup of buttercream and spread it around evenly with an offset spatula. Add more buttercream as needed to reached the desired thickness. Repeat with the second and third layer (don't forget to remove the parchment each time). Use an offset spatula to apply a thin layer of frosting to the side and top of the cake. Chill in the refrigerator for 30 minutes to set this first layer of buttercream (this is known as the crumb coat). Cover the entire cake with a final layer of frosting. Top with freshly grated nutmeg.
This cake is best when served at room temperature.
Notes
Dark rum will have a more pronounced flavor in the cake and buttercream than the spiced rum, but use what you already have. See the rest of the post for more cake tips!