1 cup granulated sugar
4 large egg whites
1 teaspoon cinnamon
2 teaspoons vanilla extract
1/4 tsp. salt
3 sticks unsalted butter cut in to 1/2 inch cubes and brought to (room temperature!)
Put the sugar, egg whites and salt in to a metal mixing bowl. Place the bowl over a saucepan of simmering water and stir constantly with a whisk until the mixture feels very warm to the touch (about 3-4 minutes)
Remove the bowl from the heat. Working with the whisk attachment or with a hand mixer, beat the meringue on medium speed until it reaches room temperature (about 5 minutes).
Add the butter cubes one at a time to beating after each addition until smooth. Once all the butter in incorporated, add the cinnamon and vanilla beating until fully incorporated.
Allow the cupcakes to cool before frosting.