Tint half of the buttercream orange. Stir melted chocolate into the rest of the buttercream until incorporated.
Use a 1/4-cup spring-loaded cookie scoop to place a small mound of buttercream on each of the cupcakes. (If you don't have a scoop, mound and smooth 1/4 cup buttercream on each cupcake.) Decorate the orange-frosted cupcakes with candy corn, placing the candy very close together. Use malted milk balls to decorate the chocolate-frosted cupcakes. Chill cupcakes in the refrigerator until ready to serve. Makes 24 cupcakes
Whisk together egg whites and sugar in a heatproof mixing bowl 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 a mixer with a whisk attachment or use a handheld mixer and beat on high until it cools and stiff peaks form, 10 to 12 min.
Reduce speed to medium-high and add butter a few tablespoons at a time. (Buttercream may curdle but will become smooth as you continue to beat it.) Add vanilla.
Use within a few hours or refrigerate for up to a week. Buttercream can be frozen for up to a month. Before using, bring to room temperature and beat on low until smooth.