Chocolate Swiss Meringue Buttercream Frosting

This is adapted from Martha Stewart's and Handled the heat's recipe. 1 recipe is enough for frosting a 2-layer cake or 24 cupcakes. I doubled recipe to make the buttercream to frost the basket cake.

Ingredients

Egg white - 5
Sugar - 1 and 1/4 cups
Unsalted butter - 1.5 cups, room temperature
Bittersweet chocolate - 9 oz, melted and cooled

Steps

1. Whisk together sugar and egg whites in a heatproof mixing bowl. Place over a pot of simmering water; Whisk gently and constantly. Heat mixture until sugar is dissolved and mixture is slightly thickened and foamy and registers 150°F on a candy thermometer or a digital thermometer, about 3 minutes.
2. Remove from heat. With an electric mixer, whisk on high speed until cool (test by touching bottom of bowl), 7 to 10 minutes.
3. Change to the paddle attachment. With mixer on medium-high speed, add butter, 2 tablespoons at a time, until combined. 

4. Beat in chocolate.