4 sticks (1 pound) unsalted butter, at room temperature
4 ounces bittersweet chocolate, melted and cooled
1/4 cup malt powder, dissolved in 2 tablespoons of hot water
12 malted milk balls, crushed
In the bowl of a standing electric mixer, combine the egg whites and sugar. Set the bowl over a pot of simmering water and whisk until the sugar is dissolved and the egg whites are just warm to the touch. Return the bowl to the mixer fitted with the whisk. Add the vanilla and beat the egg whites at high speed until firm and glossy, about 5 minutes. With the machine on, whisk in the butter a few tablespoons at a time. If the mixture begins to look curdled, continue to beat until smooth before adding more butter. Transfer 1 1/2 cups of the buttercream to a bowl and whisk in the melted chocolate. Beat the dissolved malt powder into the remaining buttercream, then beat in the milk balls.