Layering both white chocolate and chocolate buttercreams between tiers of Rich Chocolate Cake provide an element of deliciousness and graphic interest that makes our Marble Cake stand out.
Combine egg whites and sugar in a heatproof mixer bowl set over a pan of simmering water. Whisk until mixture registers 160 degrees, about 5 minutes.
Attach bowl to a mixer with the whisk attachment. Beat on high speed until cooled and thickened, about 10 minutes. Reduce speed to medium. Add butter, 1 tablespoon at a time. Reduce speed to low. Beat in white or semi-sweet chocolate, occasionally scraping down the sides and bottom of the bowl.
Raise speed to medium-high. Beat until well combined.