Make the Cake: Preheat oven to 350 degrees F. Grease three 8-inch round cake pans, line the bottoms with rounds of parchment paper, grease the parchment, then flour the insides of the pans, tapping out excess; set aside.

In the bowl of an electric mixer (or large mixing bowl if you're using a hand mixer), sift together the flour, sugar, cocoa powder, baking soda, baking powder and salt. In a medium bowl, whisk together the eggs, buttermilk, coffee, oil and vanilla.

Add the wet ingredients to the dry ingredients and mix for 2 minutes on medium speed. Scrape the sides and bottom of the bowl and mix for an additional 20 seconds (the batter will be very thin).

Divide the batter evenly among prepared pans. Bake for 20 minutes and rotate the pans in the oven. Continue to bake until a toothpick inserted into the center of one of the cakes comes out almost clean (with a few moist crumbs), about 12 more minutes. Cool the cakes (in the pans) on wire racks for 20 minutes, then carefully turn them out onto cooling racks to cool completely.

Make the Toasted Marshmallow Filling: Place the marshmallows on a baking sheet lined with aluminum foil and sprayed with nonstick cooking spray. Place on the lowest rack of oven, and broil marshmallows until nice and brown on top, keeping an eye on them the entire time so they don't burn. Remove pan from oven and gently turn the marshmallows over, and broil until the other side is golden brown.

Beat the butter and powdered sugar on low speed until blended together, about 1 minute. Add the vanilla extract and increase the speed to medium-high; beat for 3 minutes. Stop the mixer, add the Marshmallow Fluff and toasted marshmallows, and mix on the lowest speed for about 1 minute, scraping the sides of the bowl as necessary.

Make the Malted Chocolate Frosting: Beat the butter and powdered sugar on low speed until blended together, about 1 minute. Add the Ovaltine, vanilla and salt, and continue to beat on low until well combined, about 1 to 2 minutes. Add the melted chocolate and beat on medium speed until smooth, about 2 minutes. Add the heavy cream and beat on medium-high speed for another minute.

Assemble the Cake: Slice each cake layer in half horizontally so you have six cake layers. Place your first layer face-up on a cake plate and cover with one-third of the Toasted Marshmallow Frosting. Place another cake layer face-up and cover with about ¾ to 1 cup of Malted Chocolate Frosting. Repeat, alternating marshmallow and chocolate filling layers, until you come to your cake final layer, which you will place face-down. Frost the entire outside of cake with the remaining Malted Chocolate Frosting.