1. Preheat oven to 350 degrees and line three 8 inch cake pans with parchment paper and grease the sides.
2. Beat butter and sugar until light in color and fluffy, 3-4 minutes.
3. Add vanilla extract and sour cream and mix until combined.
4. Add egg whites two at a time, scraping down the sides of the bowl as needed to make sure all is incorporated and smooth.
5. In a separate bowl, combine dry ingredients. In another bowl or measuring cup, combine eggnog and water.
6. Had half of the dry ingredients to the batter and mix until combined. Add eggnog/water mixture and mix until combined, scraping down the sides of the bowl as needed. Add remaining dry ingredients and mix until smooth.
7. Divide batter evenly between the three 8 inch pans and bake for 21-24 minutes, or until a toothpick inserted comes out with a few crumbs.
8. Allow cakes to cool in pans for 2-3 minutes, then remove to cooling rack to cool completely.
9. While cakes cool, make the frosting. Beat the butter and shortening together until smooth.
8. Add half of the powdered sugar and beat until smooth.
9. Add eggnog, rum extract and nutmeg and beat until smooth.
10. Add remaining powdered sugar and beat until smooth.
11. When the cakes are cool, remove the tops of the cakes with a large serrated knife so they are flat.
12. Place the first layer of cake on cake stand. Top with one cup of eggnog buttercream and spread into an even layer.
17. Add second layer of cake and add one cup of eggnog buttercream and smooth into a smooth layer.
18. Top cake with remaining cake layer.
19. Ice the outside of the cake with the remaining buttercream. Here’s a [tutorial|https://www.lifeloveandsugar.com/2015/07/30/how-to-frost-a-smooth-cake-with-buttercream/] for getting a smooth finish and piping on the edges.
20. Store cake in fridge until ready to serve. Allow cake to sit at room temperature for 20-30 minutes before serving.
This can be made as cupcakes. Fill liners just over half way with batter and bake for 16-18 minutes, or until a toothpick inserted comes out with a few crumbs. Should make about 24 cupcakes. You should only need about half of the frosting recipe.