Heat about an inch of water in a medium sauce pot until it is just about simmering. The most important thing is that the bowl of your stand mixer should fit inside so that it is “nested” but the water does not touch the bottom of the bowl.

Whisk together the egg whites and sugar. Then place the bowl over top of simmering water and continue whisking constantly.

Cook the whites and sugar just until it reaches 120°F on an instant-read thermometer or until the sugar has dissolved and the mixture is warm to the touch. The meringue will have started to gain a bit of volume and feel lighter at this point as well.

Move the bowl to your stand mixer and fit it with the whisk attachment. Be careful and use a towel, because the bowl will be hot if you're using a gas burner! Beat on high speed until the meringue reaches stiff peaks and cools to below body temperature. This will only take 2-4 minutes, depending on your mixer.

Switch to the paddle attachment and then add all the butter. Mix on medium-low until all the butter works itself into the meringue and a silky smooth buttercream forms.

Once your buttercream comes together, stop the mixer, add all the melted but not hot chocolate, and mix on medium speed to incorporate. Switch back to the whisk and beat until light and fluffy! Frost your cakes or cupcakes, or store in the refrigerator or freezer.