In a small bowl, mix egg whites and 0.25 cup buttermilk. Set aside.

In a mixing bowl, beat butter and sugar until creamy, about 2-3 minutes. Add in dry ingredients (cake flour, baking powder, kosher salt, cinnamon, nutmeg, ground ginger) and mix until combined. Slowly add in egg white mixture. Beat in remaining buttermilk.

Divide batter evenly between two greased and floured 9-inch round cake pans. Bake in a 350 degree F oven for 25-30 minutes, or until a wooden skewer inserted into the center comes out clean.

Remove cakes from oven and let cool in pans for 10 minutes before inverting onto a wire rack to cool completely.

Preheat oven to 325 degrees F. Using a large roasting pan on the lower 1/3 of the oven, preheat the pan. Prepare a 9-inch springform pan by wrapping the bottom (outside) with a double layer of foil to prevent water seepage. Spray the pan with nonstick baking spray and line the bottom (inside) with a circle of parchment paper. Meanwhile, boil several cups of water to use later for the water bath.

In a large mixing bowl, beat softened cream cheese with granulated sugar for 2-3 minutes until creamy. Add in salt and eggs, one at a time, beating well after each addition. Beat in sour cream, heavy cream, rum flavoring, and nutmeg until just combined. Pour the cheesecake batter into the prepared 9-inch springform pan. Place the springform pan in the center of the preheated roasting pan. Carefully pour boiling water, slowly, into the roasting pan until about one inch of water comes up the sides of the springform pan (creating a water bath).

Bake the cheesecake for 45 minutes. Turn the oven off and let the cheesecake sit for an additional 30 minutes in the oven with the door slightly ajar. Remove from the oven and water bath, then cool completely on a wire rack. Once cooled, remove from the springform pan and place in the freezer for one hour, or overnight, until firm and ready to assemble the cake.

In a large mixing bowl, beat softened unsalted butter for 3 minutes until fluffy. Add in nutmeg, rum flavoring, powdered sugar, and low fat eggnog. Beat for an additional 3-5 minutes until the frosting is light and fluffy.

To assemble the cake: Place one cooled spice cake layer on a cake plate or serving stand. Evenly spread the chilled eggnog cheesecake layer over the first cake layer. Top with the second spice cake layer. Frost the entire cake with the prepared eggnog buttercream frosting. Serve and enjoy!