In the bowl of an electric stand mixer, add the egg whites and granulated sugar. Whisk them together briefly by hand until just combined (so that the egg whites don’t scramble).

Fill a sauce pan with a few inches of water and bring it to a simmer. Place the mixer bowl with the egg white mixture on top to create a double-boiler. The water should be kept at a simmer and should not touch the bottom of the bowl. This will indirectly being the eggs to a safe temperature without them becoming hard-cooked or scrambled.

Occasionally stirring, heat the egg white mixture until it reaches 170°F on a candy thermometer. The mixture will be very hot to the touch and the sugar should be dissolved.

Once the egg white mixture is hot, carefully return the bowl to the stand mixer. Fitted with the whisk attachment, beat the mixture on high speed for 8 to 10 minutes. When done, the meringue should hold shiny, medium-stiff peaks and the outside of the bowl will have returned to room temperature.

Meanwhile, melt the chocolate. You can do this by placing the chocolate in a heat-safe bowl and setting over the same saucepan of simmering water.

With the mixer on medium-low, add in the butter - a couple tablespoons at a time.

Once the butter has been mixed in, add the vanilla extract.

Stop the mixer and swap out the whisk for the paddle attachment. Turn the mixer up to medium-high speed and mix until fluffy and smooth.

Add the melted chocolate and mix until combined.

For frosting a smooth cake, you will want to eliminate the air bubbles that were whipped into the buttercream during the process. Mix on low speed for a few minutes for the silkiest, smoothest buttercream.