**Preheat the oven** Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or line them with parchment paper.

**Combine dry ingredients** In a medium bowl, whisk together flour, baking powder, baking soda, salt, nutmeg, and cinnamon (if using).

**Cream butter and sugar** In a large mixing bowl, beat softened butter and granulated sugar on medium speed until light and fluffy, about 2–3 minutes.

**Add eggs and extract** Beat in the eggs one at a time, then mix in the vanilla extract. Scrape down the sides of the bowl as needed.

**Alternate wet and dry** On low speed, gradually add the flour mixture in three parts, alternating with the eggnog in two parts (flour-eggnog-flour-eggnog-flour). Mix just until combined; do not overmix.

**Bake** Divide the batter evenly between the two prepared pans. Bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then invert onto cooling racks. Cool completely before frosting.

**Make the buttercream** In a separate bowl, beat the softened butter until smooth. Gradually add the powdered sugar, alternating with eggnog. Blend in the nutmeg, salt, and vanilla extract. If the frosting is too thick, add a tablespoon of eggnog at a time; if too thin, add more powdered sugar.

**Assemble** Place one cooled cake layer on a serving plate. Spread a layer of eggnog buttercream on top. Place the second cake layer over it, then frost the top and sides of the cake. Dust lightly with additional nutmeg if desired.