Heat your 4.5 oz (128 grams) egg whites and 7.5 oz (213 grams) white granulated sugar over a saucepan filled with simmering water.
Whisk consistently until your eggs and sugar reach 180℉ or 82℃.
Using your whisk attachment, whip your egg whites and sugar until you reach stiff peaks and until your meringue is no longer warm.
Switch to a paddle attachment. Add your 12 oz (340 grams or 3 sticks) unsalted butter 1 tablespoon at a time.
Continue to beat until your buttercream is smooth and glossy.
Add 1 tsp vanilla extract and 1 tsp salt to buttercream.
Add 5 oz of your slightly cooled ganache (It should be warm enough that you can easily add it, but not hot)
Continue to beat your buttercream until no chocolate streaks remain.
