Preheat the oven to 350F.

Whisk together boiling water and cocoa. Cover and set aside. With a mixer, cream together the butter, sugar, salt and vanilla extract. Add the eggs, 1 at a time, and continue creaming until light and fluffy. Sift together the flour, baking soda and baking powder and add this mixture, alternately with the buttermilk. Add cocoa and water mixture and mix until thoroughly combined. Scoop into cupcake pans and bake for 20 to 25 minutes.

Let cool completely before decorating.

Put the egg whites in a clean, dry mixer bowl or in another large bowl. Have a candy thermometer at hand.

Put the sugar, cream of tartar and water in a small saucepan and stir to combine. Bring the mixture to a boil over medium-high heat, cover the pan and boil for 3 minutes. Uncover and allow the syrup to boil until it reaches 242 degrees F on the candy thermometer. While the syrup is cooking, start beating the egg whites.

When the syrup is at about 235 degrees F, begin beating the egg whites on medium speed with the whisk attachment or with a hand mixer. If the whites form firm, shiny peaks before the syrup reaches temperature, reduce the mixer speed to low and keep mixing the whites until the syrup catches up. With the mixer at medium speed, and standing back slightly, carefully pour in the hot syrup, pouring it between the beater(s) and the side of the bowl. Splatters are inevitable—don’t try to scrape them into the whites, just carry on. Add the vanilla extract and keep beating the whites at medium speed until they reach room temperature, about 5 minutes. You should have a smooth, shiny, marshmallowy frosting.