• 3/4 cup sugar
• 3 large egg whites
• 1 cup (2 sticks) plus 6 tablespoons unsalted butter, room temperature
• 3 ounces semisweet chocolate, melted and cooled

1. Combine sugar and egg whites in the bowl of an electric mixer fitted with the whisk attachment.
2. Place over a pot of simmering water; whisk until hot and sugar is completely dissolved, about 3 minutes.
3. Transfer bowl to mixer stand, and beat on high speed until cooled, 7 to 10 minutes.
4. Replace whisk with paddle attachment, and set mixer on medium-high speed; add butter, 1 to 2 tablespoons at a time, until incorporated.
5. Beat in chocolate.
6. If not using immediately, refrigerate buttercream in an airtight container for up to a week.
7. Bring to room temperature, and rebeat or stir vigorously before using.

Martha says, "If you live in a warm climate or you want to display your cupcakes for an extended period of time, use half solid vegetable shortening and half butter, which will result in a more stable frosting."