To make the chocolate cake:

1. Prepare two 8-inch cake pans with parchment paper circles in the bottom, and grease the sides. Preheat oven to 300°F (148°C).
2. Add the dry ingredients to a large bowl and whisk together.
3. Add the eggs, milk, oil, and vanilla extract to the dry ingredients and mix until well combined.
4. Add the hot water and mix until well combined.
5. Divide the batter evenly between the cake pans and bake for 45-50 minutes, or until a toothpick comes out with a few crumbs.
6. Remove cakes from oven and allow to cool for 2-3 minutes, then transfer to cooling racks to cool completely.

To make the hot chocolate frosting:

1. Dissolve the hot chocolate powder into the hot water or milk. It’ll be fairly thick. Set aside.
2. Add the butter and shortening to a large mixer bowl and beat until smooth.
3. Add about half of the powdered sugar and mix until well combined and smooth.
4. Add the hot chocolate mixture and cocoa powder and mix until well combined and smooth.
5. Add the remaining powdered sugar and mix until well combined and smooth.
6. Add additional water or milk as needed to get the right consistency.

To make the marshmallow filling:

1. Add the butter to a mixer bowl and beat until smooth.
2. Add the powdered sugar and beat until well combined and smooth, scraping down the sides of the bowl as needed.
3. Stir in the marshmallow cream. Set filling aside.

To build the cake:

1. Remove cake domes from the top with a large serrated knife, if needed. Divide each cake into two layers, so you have four total layers.
2. Place the first layer of cake on a cake plate or cardboard cake circle. Add half a cup of hot chocolate frosting and spread into an even layer.
3. Pipe a dam around the outer edge of the cake, then fill it with about 3/4 cup of marshmallow filling. Spread into an even layer.
4. Place the second layer of cake on top and add another layer of hot chocolate frosting and marshmallow filling.
5. Repeat with the third layer of cake and filling, then top the cake with the final layer.
6. Spread a light crumb coat around the sides of the cake, then refrigerate for about an hour to let the filling firm up.
7. After refrigerating, frost the outside of the cake. (Feel free to use a tutorial for frosting a smooth cake with buttercream.)

To make the white chocolate ganache:

1. Add the white chocolate chips to a medium-sized bowl.
2. Heat the cream just until it begins to boil, then pour over the white chocolate.
3. Whisk the chocolate mixture so it begins to melt. Heat for another 10 seconds and whisk to continue melting. If needed, heat another 10 seconds.
4. Drizzle the white chocolate around the top edge of the cake, using a squeeze bottle, spoon, or another method.
5. Fill in the top of the cake with the ganache and spread evenly.

To create the ring of marshmallows:

1. First, make the vanilla buttercream. Add the butter to a mixer bowl and beat until smooth.
2. Add half of the powdered sugar and beat until smooth.
3. Add a tablespoon of water or milk and the vanilla extract and beat until smooth.
4. Add the remaining powdered sugar and beat until smooth, adding additional water or milk as needed for the right consistency.
5. Pipe a ring of vanilla buttercream around the top edge of the cake, then add mini marshmallows and marshmallow bits.
6. Refrigerate the cake until ready to serve. Let it sit out for a few hours before serving to come closer to room temperature. The cake is best when stored well-covered for 2-3 days.