Ingredient quantities are listed in the recipe card below.
Step 1: Whisk together the egg whites and sugar in the bowl of a stand mixer. Place over a pot of simmering water and bring to 160 degrees F.
Step 2: Transfer the bowl to a stand mixer and whisk until soft peaks form. Add the salt & vanilla and continue whisking until stiff peaks form. Turn the mixer down to low speed and wait until it drops down to at least 85 degrees F.
Step 3: Once the meringue has reached at least 85 degrees F, add the room temperature (but on the cooler side) butter a few cubes at a time.
Step 4: Once all of the butter has been added, it may look curdled, but continue mixing and it will come together.
Step 5: Lastly, switch to a paddle attachment and mix on low speed until silky smooth.
Step 6: If desired, divide the buttercream into three parts, and add cocoa powder to one, espresso powder to one, and leave the last vanilla.