Make the Eggnog Cake:
- Preheat oven to 350. Butter and lightly flour three 8″ round pans. Line with parchment paper circles.
- In large bowl whisk together the flour, baking powder, salt, cinnamon and nutmeg. Set aside.
- In the bowl of a stand mixer, beat butter, granulated sugar and brown sugar on medium-high using the paddle attachment. Beat until pale and fluffy, about 4 minutes.
- With the mixer on low, add eggs one at a time. Mix well after each egg. Scrape down bowl as needed.
- Add vanilla extract. Mix to combine.
- In a separate measuring cup, combine milk and vegetable oil.
- Add the flour mixture in three batches, alternating with the milk/oil mixture, beginning and ending with the flour mixture. Do not over mix. Stop the mixer and scrape down the bowl.
- Divide batter evenly between the three 8″ round pans.
- Bake for 28-30 minutes, until toothpick inserted into center comes out clean.
- Let cool in pans for 10 to 15 minutes, then remove to wire rack to finish cooling.
Make the Eggnog Buttercream:
- Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.
- Place bowl over a pan of simmering water to create a double-boiler. Whisking constantly, heat the egg mixture until it registers 160 degrees on a candy thermometer. Carefully transfer the bowl onto the stand mixer.
- Using the whisk attachment, beat the egg white mixture on high speed for 8 to 10 minutes until the bowl is no longer warm to the touch and the meringue is fluffy, glossy and holds a stiff peak.
- Switch to the paddle attachment. With the mixer on low, slowly add cubed butter and mix until incorporated.
- Add Holiday Nog, cinnamon, nutmeg and vanilla. Continue mixing on low to med-low until silky and smooth, about 3 to 5 minutes.
Assemble the Cake:
- Place one layer of cake onto serving plate or cake stand and top with about one cup of frosting.
- Repeat with second cake layer, more frosting and then last layer of cake. Crumb coat and frost cake with remaining frosting.
- Decorate with piped buttercream leaves, broken cinnamon sticks and whole nutmeg, if desired.