For the cake: Preheat the oven to 350 degrees F. Butter and flour three 9-inch cake pans and three 6-inch cake pans.
Cream the butter and granulated sugar in a stand mixer. Add the eggs 1 at a time, mixing well after each addition.
Combine the flour, cocoa, baking soda and espresso in a large bowl. Combine the milk, sour cream and coffee in a separate large bowl. Alternate adding the wet and dry ingredients to the mixer until combined and a light fluffy batter is reached.
Divide the batter among the prepared cake pans and bake until a cake tester inserted in the center of the cakes comes out clean, 22 to 25 minutes. Let cool in the pans for 5 minutes, then transfer to a wire rack to cool completely, 15 to 20 minutes.
For the chocolate ganache: Combine the chocolate chips, cream and milk in a large pot and heat until the chocolate melts and the mixture is hot, stirring to combine; do not allow to boil.
To temper the egg yolk, put it in a large bowl and scoop a ladleful of the hot milk mixture into it, whisking constantly. Keep adding the rest of the hot milk mixture, a ladleful at a time, until completely combined. Whisk until the ganache is silky and shiny. Refrigerate until set and spreadable, 1 to 2 hours. (It's best if it's allowed to sit overnight to set.)
For the marshmallow buttercream: Mix the butter, confectioners' sugar, marshmallow creme and vanilla extract in a stand mixer on medium until light and fully combined.
For the cinnamon-nutmeg buttercream: Beat the butter, confectioners' sugar, milk, cinnamon and nutmeg in a stand mixer with the paddle attachment on high until fully combined, light and airy
To assemble: Top a 9-inch cake layer with 1 cup of the cinnamon-nutmeg buttercream. Place another 9-inch cake layer on top of it, then top with 1 cup of the chocolate ganache. Continue alternating cake layers with buttercream and ganache layers, leaving the very top bare. Cover the whole cake in a layer of the marshmallow buttercream.
This recipe makes a large 2-tiered cake. For one 9-inch cake, divide the recipe in half.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.