Place the sugar and water in a small saucepan over medium heat. Cover the pot for the first 2 minutes to help prevent crystallization. You will heat the sugar syrup to 240°F.

While the sugar heats, place the egg whites in the bowl of a stand mixer fitted with a whisk attachment. When the sugar syrup hits 200°F, begin whipping the egg whites on low. Once they start to foam, add the salt and cream of tartar.

Bump the speed up to medium (4 on a KitchenAid) and beat until you reach soft peaks. The whites should be ready right when your sugar syrup is ready. If they whip up quicker, just bump the speed to the lowest speed to prevent them from over whipping.

Once the sugar is 240°F, the egg whites should be at soft peaks. Slowly and carefully drizzle the syrup into the bowl, being careful to avoid the whisk. Continue to beat the meringue on a medium speed until it forms thick, glossy peaks, about 10- 15 minutes.

When the meringue is ready, you can begin adding the butter one tablespoon at a time. Start with the cold stick of butter to bring the temperature of the meringue down. Wait about 10 seconds between additions. The mixture will deflate and look loose, but it will thicken up again.

Once the butter has been added, scrape down the bowl and whip on a medium high speed for 3 minutes. If it still looks soupy, place the bowl in the fridge for about 5- 15 minutes, then whip again. Scrape the bowl down, add vanilla, and whip to combine. Taste the buttercream. If it tastes too buttery or thick, whip on a medium high speed for a few more minutes.

Place the finely chopped chocolate and cocoa powder (optional) in a heat proof bowl and melt over a double boiler or in the microwave just until smooth. Cool the chocolate slightly (you can do this while the meringue whips or while you add the butter). Add a large dollop of the buttercream to the chocolate mixture and stir until combined. Switch to a paddle attachment, add the chocolate mixture to the rest of the buttercream, and beat to combine. If your buttercream is too warm/ loose, chill for up to 30 minutes, then beat again.