Preheat oven to 350F, grease two 8″ cake rounds. Dust with cocoa powder and line bottoms with parchment.
Place all dry ingredients (flour, cocoa powder, baking soda, baking powder, sugars and salt) into the bowl of a stand mixer. Mix until well combined.
In a medium bowl, whisk eggs, oil, buttermilk, and vanilla. Very slowly pour in hot hot chocolate while whisking constantly.
Add wet ingredients to dry and mix on medium for 2-3 minutes. Batter will be very thin.
Pour batter evenly into prepared pans. Bake for approximately 30 minutes or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10 minutes then turn out onto wire rack to cool completely.
Make the Marshmallow Fluff:
In a medium saucepan, combine water, sugar, and corn syrup. Stir to combine until sugar is dissolved and do not stir from this point forward. Insert a candy thermometerand cook on med-high.
Place egg whites and cream of tartar into the bowl of a stand mixer*.
When the sugar syrup reaches 225F, start whipping egg whites until they reach soft peaks (3-4 minutes).
When the sugar syrup reaches 240F, remove from heat, turn mixer to medium and very slowly pour in the sugar syrup into the whites in a thin stream. Be careful as it will be hot!
Turn the mixer to med-high and whip until thick, glossy (7-8 minutes).
Add vanilla and whip until fluff has cooled.
Make the Chocolate Frosting:
Place egg whites and sugar into the bowl of a stand mixer*, whisk until combined.
Place bowl over a pan of simmering water to create a double-boiler. Whisking constantly, heat the egg mixture until it registers 160 degrees on a candy thermometer. Carefully transfer the bowl onto the stand mixer.
Using the whisk attachment, beat the egg white mixture on high speed for 8 to 10 minutes until the bowl is no longer warm to the touch and the meringue is fluffy, glossy and holds a stiff peak.
Switch to the paddle attachment. With the mixer on low, slowly add cubed butter and mix until incorporated.
Add vanilla, chocolate and whip on medium-high until silky and smooth, about 3 to 5 minutes.**
Assemble the Cake:
Place one layer of cake onto a cake stand or serving plate. Pipe a dam around the edge with the chocolate frosting and fill with approximately 1 cup of marshmallow fluff. Spread evenly.
Place remaining layer on top and crumb coat the cake. Refrigerate for 20 minutes.
Frost the cake with the chocolate frosting, smoothing the sides and top. Place back in the refrigerator for 20 minutes to set.
Pipe a scallop pattern (using a large round piping tip and a small offset spatula, following this technique) on top and bottom edges using the remaining marshmallow fluff. Top with chocolate shavings if desired.
* Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk are completely grease free or your meringue may not whip up.
** The buttercream may look like it’s curdled at some point. Keep mixing until it is completely smooth.