Makes either a 2-layer 8" or 9" cake, or a 3-layer 6" cake. Baking times will vary and are indicated for each size below.
Preheat oven to 350F and prepare cake pans by lightly but thoroughly greasing and flouring.
In bowl of KitchenAid mixer (or in large bowl if using hand-mixer) whisk together sugar, flour, cocoa powder, baking powder, baking soda, and salt.
With mixer on low speed, stir in canola oil and water.
Increase speed to medium and add eggs, one at a time, beating between each addition until well-combined.
Stir in vanilla extract.
Gradually add hot chocolate to bowl, stirring in manually. Return to KitchenAid (or to hand-mixer) and stir on low-speed until well-combined (pause occasionally to scrape sides and bottom of bowl to make sure ingredients are combined completely).
Evenly divide batter into prepared pans (do not fill more than 3/4 full, if you have excess batter, discard).
If using 8" or 9" pans: bake on 350F for 25-30 minutes (until toothpick inserted in center comes out clean or with few crumbs).
If using 6" pans: bake on 350F for 40-45 minutes (until toothpick inserted in center comes out clean or with few crumbs).
Once finished baking, allow cakes to cool in pans for 15 minutes. Run a butterknife around the inside rim of the pan to loosen the cakes and then invert onto cooling racks to remove from pans and allow to cool completely before frosting.
Measure out your milk and set aside.
While cakes are cooling, prepare frosting by beating butter on medium speed (with KitchenAid or hand mixer) until well-creamed.
Add cocoa powder and 1 cup of powdered sugar, beat on low speed until it begins to combine.
Add approximately half of the milk and the vanilla and beat on low speed until ingredients are creamed together.
Add the remaining powdered sugar, stir, and then add remaining milk, stirring until ingredients are creamed together.
Be sure to scrape sides and bottom of bowl to ensure ingredients are well-combined.
Set aside chocolate frosting and prepare marshmallow buttercream.
Cream together butter and marshmallow fluff.
Gradually add powdered sugar until combined.
Once cakes are completely cooled, prepare by icing each layer with chocolate, applying the chocolate most heavily 1" around the edges and creating an indent or "nest" in the center for the marshmallow fluff.
Spread or pipe marshmallow fluff into the indent created with the chocolate frosting, and then top with the next layer and repeat.
Ice the entire outside of cake with the chocolate frosting and pipe or spread marshmallow buttercream on the center of the top.
Just before serving, top with marshmallows for decoration (if desired) and serve.
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