Preheat the oven to 350 degrees. Spray an 8-inch round cake pan with cooking spray. Line the bottom of the pan with parchment paper, then generously spray the pan with cooking spray again. Set pan aside.

Beat the cream cheese, sugar, and flour together with an electric mixer set to medium speed. Beat in eggs and egg yolks one at a time, mixing until eggs are just incorporated. Scrape down the sides and bottom of the bowl. Turn mixer to low and beat in the sour cream, eggnog, rum flavor, vanilla, and nutmeg until smooth.

Pour batter into the prepared pan and smooth the top. Set pan in a roasting pan (or any large baking pan) and carry over to the oven. Pour a couple inches of water into the pan, then carefully transfer to the oven. Bake 32 to 38 minutes, until the center of the cheesecake barely wobbles. Remove from the oven and immediately run a knife between the cake and the edge of the pan. Cool cheesecake to room temperature. Once cooled, cover loosely with plastic wrap and refrigerate overnight.

After the cheesecake has refrigerated, dip the pan in a bowl of hot water for about 15 seconds. Run a knife around the edge of the cake again, then set a large plate or a cake circle on top of the pan. Invert the cake onto the plate. Keep refrigerated until you assemble the cake.

Preheat oven to 350 degrees. Spray 2 8-inch round cake pans with cooking spray and line the bottoms with parchment paper. Spray the paper with cooking spray and lightly dust with flour. Set pans aside.

Melt the butter in a medium saucepan set over medium heat. Add the water and bring mixture to a boil, stirring occasionally. Remove from the heat and stir in chopped chocolate until it melts. Pour into a medium mixing bowl and cool for 10 minutes, then stir in the buttermilk, eggs, and vanilla. Set aside.

Combine the flour, sugar, baking soda, and salt in a large mixing bowl and whisk to combine. Gradually stir in the chocolate mixture and stir until well combined. Divide evenly between the prepared pans. Bake 24 to 30 minutes, until a cake tester inserted into the center of the cakes comes out clean. Cool in pans on wire racks for 10 minutes, then carefully invert the cake layers onto the racks to cool completely.

Place the chocolate in a microwave-safe bowl and heat at 50% power in 15-second intervals, stirring after each interval, until chocolate is melted and smooth. Set aside to cool.

Beat the butter in a stand mixer fitted with the paddle attachment until light and creamy, about 2 minutes. Turn mixer to low and beat in 5 cups of the powdered sugar, 2 tablespoons of the milk, and the vanilla and salt until well combined. Add the cooled white chocolate and mix on low until incorporated. Turn mixer to high and beat for 4 minutes, until very light and fluffy. Add additional powdered sugar to thicken frosting or additional milk to thin it out to a spreadable consistency.

If your cakes have domed, use a large serrated knife to trim them so they’re even.

Place one of the layers on a cake circle or cake stand and spread a thin layer of frosting over the top. Carefully top with the cheesecake layer. Spread a thin layer of frosting over the cheesecake and top with remaining cake layer. If your cheesecake layer is wider than your cake layers, use a sharp knife to trim off the excess and make them even.

Frost the cake with the remaining frosting and decorate as desired. Serve.