INGREDIENTS
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1 teaspoon good-quality instant espresso powder
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3 tablespoons boiling water
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1/2 cup (1 stick) unsalted butter, softened
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1/2 cup granulated sugar
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1 teaspoon pure vanilla extract
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1/2 teaspoon coarse salt
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1 large egg
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1/2 cup unsweetened Dutch-process cocoa powder
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1 3/4 cups all-purpose flour
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1 large egg yolk
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Sanding sugar, for sprinkling