• 1 ½ cups granulated sugar (300g)• 5 large egg whites about ¾ cup/177ml• ½ teaspoon table salt• 2 cups unsalted butter softened but not warm (approximately 60F/15C) (453g)• 12 oz dark or bittersweet chocolate chopped (60-72% cacao) (340g)• 1 ½ teaspoons vanilla extract
Prepare a double boiler. In the bowl of your double boiler, whisk together granulated sugar, egg whites (make sure there’s no yolk mixed in!) and salt until completely combined.
Fill a medium-sized saucepan with about 1 ½” of water and make sure your egg whites bowl rests in the saucepan without touching the water. Remove bowl, turn heat to medium.
Once simmering, place bowl on the saucepan and whisk the contents constantly until the sugar is dissolved and/or the mixture reaches at least 160F (71C).
Remove the bowl and transfer it to a stand mixer. Fit mixer with a whisk attachment and begin to beat the mixture on medium speed. Set aside and prepare your chocolate.
Place chopped chocolate in a microwave safe bowl and melt in 20-second intervals, stirring well in between, until chocolate is completely smooth and melted.
Slowly increase speed to high and beat until meringue is thick and glossy and you have reached stiff peaks. Once you've reached stiff peaks, switch to the paddle attachment.
Set mixer speed to medium and add butter, one Tablespoon at a time, not adding the next Tablespoon until the previous one is completely mixed into the meringue.
Then, check your chocolate temperature. It should smooth, melted and no longer feel warm to the touch. If so, slowly drizzle it into your buttercream. Mix on low speed.