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Macaron Filling

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In the bowl of an electric mixer, whisk egg whites and sugar. Set mixer bowl over a saucepan of simmering water and heat mixture, whisking often, until it feels warm to the touch and sugar is dissolved, 3 to 5 minutes.
Transfer bowl to the mixer, and fit with the whisk attachment. Whip on high speed until mixture is stiff and shiny, 3 to 5 minutes. Add butter, one piece at a time, and continue mixing until butter is thoroughly incorporated. The filling can be kept, covered and refrigerated, up to 1 week. Bring to room temperature before stirring.
Variations: To make hazelnut-honey filling: In a small bowl, combine 1/2 cup of macaron filling with 1/3 cup finely ground hazelnuts and 2 tablespoons good-quality honey.