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Mix sugar and lemon zest in a small bowl.
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Whisk yolks with 5 tblsps lemon sugar in small bowl until thick and pale.
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Bring butter and juices to boil in small saucepan, stirring frequently.
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Gradually whisk into yolk mixture.
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Return to saucepan and whisk over medium-low heat until mixture resembles thick custard, about 4 minutes.
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Do not let boil.
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Pour into bowl and cool completely (can be prepared up to 1 day ahead and refrigerated. Bring to room temperature and whisk before continuing.).
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Position rack in center of oven and preheat to 350F.
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Butter 2 ½ quart soufflé dish.
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Sprinkle 1 tblsp sugar over bottom and up sides of dish.
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Beat whites until soft peaks form.
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Gradually add remaining lemon sugar and beat until stiff but not dry.
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Fold ¼ of whites into yolk mixture to lighten.
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Gently fold in remaining whites.
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Transfer to prepared dish.
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Bake until soufflé rises but center is not firm to touch, about 20 minutes.
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Sift icing sugar over top.
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Serve immediately, passing Warm Caramel Sauce separately.
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For Warm Caramel Sauce (about 1 ½ cups): Cook sugar and water in medium saucepan over low heat, stirring until sugar dissolves.
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Increase heat and boil without stirring until sugar turns a deep golden-brown.
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Reduce heat to low.
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Add whipping cream, salt and vanilla extract and stir until smooth.
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Can be prepared 1 week ahead.
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Cool, cover and refrigerate.
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Rewarm over low heat before serving.