5 egg whites
1 cup granulated (NOT powdered!) sugar
3 sticks unsalted butter
2 tbsp pure vanilla extract
1 cup chocolate (chips or otherwise), melted and cooled slightly – not hardened.
Mix egg whites and sugar in a *clean* metal mixing bowl, and place over a pot of simmering water on your stovetop. Whisk occasionally until it hits 160 degrees.
Move egg mixture to your stand mixer and whip on high (using the whisk attachment) until stiff peaks form, and mxture is relatively cool. While waiting, cut up the butter into chunks.
When Meringue has reached the stiff peaks stage, switch to low speed and add the butter a chunk at a time. Add vanilla.
Once vanilla and butter is incorporated into the mix, turn speed back up to high and whip until you have a smooth buttercream. It will go through some weird stages before this point – soupy, maybe curdled. Don’t worry! It will come together!
Once frosting is ready, turn speed back down to low. Making sure that the melted chocolate is no longer hot, slowly add it to the mixing bowl, pouring down the side of the bowl. Once all of the chocolate is in the bowl, you can speed up the mixer a little until it is all incorporated, and smooth.