In a small saucepan over medium heat, whisk together the milk and hot cocoa mix. Bring to a simmer, then set aside and cool to room temperature before continuing on with this recipe.

Preheat the oven to 350ºF (177ºC) and prepare three 6-inch or two 8-inch cake pans by spraying the sides with baking spray and fitting the bottoms with a parchment cake circle.

Place all of the dry ingredients into the bowl of a stand mixer fitted with the paddle attachment and stir on low for 30 seconds to fully combine them. Add the vegetable oil, eggs, vanilla, and hot cocoa and mix on low until just combined. Ensure that the hot water has been heated to a simmer, then with the mixer still on low, add the hot water in a slow stream. Turn the mixer to medium and beat until smooth, about 2 minutes. The batter will be very thin.

Pour into prepared cake pans no more than ⅔ full and bake for 32-36 minutes, or until a wooden toothpick inserted comes out clean. Cool completely before frosting.

In a stand mixer fitted with a paddle attachment, whip butter on medium speed until creamy and light in color, about 5 minutes. Scrape down the bowl and paddle and add half of the powdered sugar, then mix on low speed until combined. Add the rest of the powdered sugar, cocoa powder, hot cocoa, vanilla, and salt. Mix on low speed until combined and smooth, scraping down the bowl and paddle as needed.

Fill a medium saucepan with an inch or two of water and bring it to a simmer over medium heat. Whisk together the egg whites, granulated sugar, and cream of tartar in a heatproof bowl, then place it on top of the simmering saucepan. Make sure that the bottom of the bowl does not touch the water.

Whisk the egg white mixture continuously until the sugar and cream of tartar are dissolved, 3-4 minutes. It will thin out and be very frothy on top.

Remove the mixture from the heat and add it to a stand mixer fitted with the whisk attachment. Add the vanilla, then beat on high speed until stiff, glossy peaks form, about 3-5 minutes.

Once the cake layers are completely cool, level them to your desired height. Pipe a ring of hot cocoa buttercream around the edge of the first cake layer, then fill the middle halfway with marshmallow creme filling before filling the rest of the space with hot cocoa buttercream. Add the next cake layer on top and repeat the filling process before adding the final cake layer on top, upside down so that the bottom of the cake is facing up. Crumb coat with the hot cocoa buttercream and refrigerate for 20 minutes.

Use the rest of the hot cocoa buttercream to frost a smooth buttercream finish, keeping the top edge 'unfinished'. Use clean kitchen scissors or a paring knife to slice the mini marshmallows in half and place them onto the buttercream finish in an ombrè layout. Finish the design by adding the rest of the marshmallow creme on top, then piling marshmallows of various sizes onto it and dusting with cocoa powder.