INGREDIENTS
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For the Cupcakes: 2 1/4 cups cake flour, like swan's down brand
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1 tablespoon baking powder
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1/2 teaspoon kosher salt
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1 1/4 cups whole milk, room temperature
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4 large egg whites, room temperature
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lemon zest, from one medium lemon
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lime zest, of one medium lime
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1 1/2 cups granulated sugar
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1 stick unsalted butter, (8 tablespoons), at room temperature
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2 teaspoons pure vanilla extract
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For the Buttercream: 3 sticks unsalted butter, softened (1 1/2 cups)
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1 1/2 pounds powdered sugar, sifted (24 ounces)
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2 teaspoons pure vanilla extract
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lemon zest, from 1 medium lemon
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lime zest, from 1 medium lime
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1 tablespoon fresh lemon juice
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1 tablespoon fresh lime juice
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1-2 tablespoons milk, if necessary