Preheat oven to 350F, grease three 6" round baking pans and dust with cocoa powder. Line bottoms with parchment.

Place all dry ingredients into the bowl of a stand mixer fitted with a paddle attachment. Stir to combine.

In a medium bowl whisk all wet ingredients (pour water in slowly as not to cook the eggs if very hot).

Add wet ingredients to dry and mix on medium for 2-3 mins. Batter will be very thin.

Pour evenly into prepared pans. I used a kitchen scale to ensure the batter is evenly distributed.

Bake until a cake tester comes out mostly clean. A total of 30-35mins.

Cool 10 minutes in the pans then turn out onto a wire rack to cool completely.

Place egg whites and dark brown sugar into the bowl of a stand mixer, whisk until combined.*

Place bowl over a pot with 1-2" of simmering water and stir constantly with a whisk until the mixture is hot and no longer grainy to the touch or reads 160F on a candy thermometer (approx. 3mins)

Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).

Switch to paddle attachment. Slowly add cubed butter and mix until smooth.**

Add dulce de leche and whip until smooth.

Place chopped chocolate and cream into a microwave safe bowl. Stir to combine. Microwave for 20 seconds, stir. Microwave in 10 second intervals, stirring in between, until ganache is smooth and silky. Set aside to cool completely and thicken slightly before using on cake.

Place one layer of cake on a cake stand or serving dish. Top with about ⅔ cup buttercream. Repeat with remaining layers. Frost and smooth the outside with a thin crumb coat. Chill for 20mins.

Using a small spoon, place dollops of ganache around the top edges of the cooled cake, allowing some to drip down. Fill in the top of the cake with more ganache and spread evenly with an offset spatula.***

Top with decorative dollops of frosting (I used Ateco tip 825). Drizzle slices with dulce de leche if desired.