This is an excellent addition to my Mexican Chocolate cupcakes, but I could also see it pairing nicely with a yellow or white cake cupcake.
3 large egg whites, at room temperature
3/4 c. sugar
1 c. unsalted butter, cut into 16 pieces, at room temperature
3/4 t. cinnamon
In a double boiler, using simmering water, combine the egg whites and sugar in the upper bowl. Heat the mixture, whisking constantly, until the sugar has completely dissolved and the mixture is very warm to the touch, about 2 minutes. Remove the bowl from the saucepan. Use an electric mixer on high speed to beat the egg white mixture until it's fluffy and holds stiff peaks, about 6 minutes.
Turn the mixer to medium-low and add the salt and the butter, a few pieces at a time, beating well after each addition. If it separates at all, keep beating, but this time on high until smooth and creamy. Add the cinnamon and beat until combined.
~Original recipe from Cupcakes