INGREDIENTS
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12 tablespoons unsalted butter
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3/4 cup sugar
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1 large egg
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1 tablespoon finely grated lemon zest
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1 teaspoon pure vanilla extract
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2 1/4 cups all-purpose flour
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1/4 teaspoon salt
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2 large egg yolks, lightly beaten and separated in equal portions into 2 or more bowls or cups
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food coloring