INGREDIENTS
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6 tablespoons (85 g) unsalted butter, softened
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1 cup (230 g) packed dark brown sugar
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1 teaspoon (5 ml) pure vanilla extract
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1 large egg, at room temperature
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1 egg yolk, at room temperature
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3/4 cup + 2 teaspoons (115 g) all-purpose flour
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6 tablespoons (38 g) unsweetened Dutch-processed cocoa powder
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1/2 teaspoon (3 g) baking soda
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1/2 teaspoon (3 g) baking powder
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1/2 teaspoon (5 g) kosher salt
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3/4 cup (180 ml) buttermilk, at room temperature
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2 tablespoons (30 ml) mayonnaise
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5 large fresh egg whites (150 g)
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1-1/4 cup (250 g) superfine granulated sugar
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1-1/2 cups (340 g) (3 sticks) unsalted butter, softened, cut into cubes
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2 teaspoons (10 ml) pure vanilla extract
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pinch of salt