Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.**
Place bowl over a double boiler on the stove and whisk constantly until the mixture is hot and no longer grainy to the touch (approx. 3mins). Or registers 160F on a candy thermometer.
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 cooled melted chocolate and whip until smooth.

Assembly:

Place a layer of cake on a cake board or plate. Top with approx. 1 cup of buttercream and spread evenly. Place second layer on top and do a thin crumb coat on the cake. Chill for 20mins.
Place 1 cup of frosting on top and spread evenly. Smooth the sides and flatten the top, then use a large offset spatula to do a swirl pattern on top.
Using a piping bag fitted with a 1M tip, pipe rows of frosting around the cake, starting at the bottom. Apply even pressure as you rotate your turntable. Continue working your way up the cake to the top, making sure the seams all align at the back.
Decorate the top with chocolate curls, pearls, and flakes if desired.