"This recipe is an accompaniment for Old-Fashioned Potato Salad...."
INGREDIENTS
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3 large egg yolks
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1 tablespoon all-purpose flour
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1 tablespoon sugar
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1 teaspoon salt
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1 teaspoon dry mustard
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1 1/4 cups whole milk
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1 tablespoon cider vinegar
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3 tablespoons unsalted butter
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Large bowl of ice and cold water