Marshmallow Frosting (which most southerners will know as Seven-Minute Icing) is my favorite for chocolate and coconut cupcakes. It makes a beautiful glossy presentation that can be piled high, but it must be used immediately. It will keep for most of the day it is made (if not too humid), but the egg whites will begin to break down after about 12 hours.
When I was growing up, we used to fight to lick the bowl after my mom made this icing. On really humid days she couldn't make it (which was most of the time), so we learned to appreciate it when we had it.
8 large egg whites
2 cups sugar
1/2 teaspoon cream of tartar
2 teaspoons vanilla
Place egg whites, sugar, and cream of tartar in the heatproof bowl of an electric mixer. Set over a saucepan with simmering water. Whisk constantly until sugar is dissolved and whites are warm to the touch - approximately 3 to 4 minutes.
Transfer bowl to electric mixer fitted with the whisk attachment, and beat, starting on low speed, gradually increasing to high, until stiff glossy peaks form (5 to 7 minutes). Add vanilla, and mix until combined. Use immediately.