Whisk egg whites and sugar in a heatproof bowl. Place over a pot of simmering water (double boiler) and whisk constantly for 3 minutes, until sugar dissolves and the mixture reaches 160°F.

Step 4: Whip the Buttercream
Move the bowl to a stand mixer and whisk on medium-high for 5–10 minutes until stiff, glossy peaks form and the bowl is cool. Switch to the paddle attachment and gradually add cubed butter until smooth. Fold in the cooled, melted chocolate.

Step 5: Assemble & Decorate
Place the first cake layer on a plate and spread 1 cup of frosting on top. Add the second layer. Apply a thin crumb coat over the entire cake and chill 20 minutes. Finish with a final layer of frosting, piping decorative swirls or rows with a 1M tip. Garnish with chocolate curls or sprinkles.