Construct a double boiler by fitting the bowl of a stand mixer into a saucepan with about an inch of water. The bowl should not touch the water.

In the bowl, whisk together egg whites and sugar and bring the water to a simmer. Continue whisking until sugar is completely dissolved and mixture is quite hot (160°F).

Place bowl on stand mixer fitted with the whisk attachment, add vanilla, and whip on high until completely cool, about 10 minutes.

Meanwhile, melt chocolate in a heatproof bowl, either by setting over simmering water or by microwaving in 20-second increments and stirring in between. Add cinnamon and stir in. Let cool.

Once the egg white mixture is cool, decrease the speed of the mixer to low, and add the butter in 1-2 tablespoon increments, waiting until the butter is incorporated before adding the next. Continue until the frosting looks light and fluffy.

With the mixer still on low, add the chocolate and mix until uniform, scraping the bottom of the bowl to ensure everything is mixed.