• Make Buttercream: Whisk together 5 large egg whites and 1¼ cups sugar in the heatproof bowl of an electric mixer over a pan of simmering water until sugar has dissolved, about 5 min. Test by rubbing the mixture between your fingers-you shouldn't feel any sugar crystals.
• Transfer bowl to the mixer stand with the whisk attachment and beat on high until mixture cools and stiff peaks form, 10 to 12 min.
• Reduce speed to medium high and add 1 teaspoon vanilla extract and 4 sticks unsalted, softened butter a little at a time. (Buttercream may curdle but will become smooth as you continue to beat it.)
• Use within a few hours or refrigerate up to a week. Before using, bring to room temperature and beat on low until smooth.

To Top Cupcakes

• Divide buttercream into thirds. Color one-third orange and one-third yellow. Fit a large piping bag with a #826 open-star tip. Spoon orange, yellow and white buttercream into the piping bag side by side (not on top of one another).
• Pipe onto a plate until all three colors appear, then pipe swirls onto each cupcake. Cupcakes can be refrigerated up to a day.