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