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1
Preheat oven to 400 degrees.
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2
Butter the inner surface of a 6-cup mold; coat the sides evenly with granulated sugar.
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3
Rub the sugar lumps over the orange to extract the oil; set aside. Beat the flour in the saucepan with a bit of the milk until well-blended. Beat in the rest of the milk and the sugar. Mash the sugar lumps, grate the orange peel and beat in to the milk and sugar mixture. Stir over moderately-high heat until mixture thickens and comes to a boil. Remove from heat and beat for 2 minutes to cool slightly.
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4
Separate one egg, dropping the white into a bowl, and the yolk into the center of the sauce. At once beat the yolk into the sauce with a wire whip. Continue with the rest of the eggs, one by one.
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5
Beat in half the butter. Clean sauce off sides of pan with a rubber spatula. Dot top of sauce with the rest of the butter to prevent a skin from forming on the surface.
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6
Beat the egg whites and salt together until soft peaks form. Sprinkle on the sugar and beat until stiff peaks are formed.
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7
Beat the vanilla and orange liqueur into the sauce base. Stir in a fourth of the beaten egg whites. Delicately fold in the rest.
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8
Turn the souffle mixture into the prepared mold, leaving a space of at least 1 1/4 inches between the top of the souffle and the rim of the mold. If the mold is too full, the souffle will spill over as it rises.
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9
Place the mold in the middle level of the preheated over and immediately turn heat down to 375 degrees. In 20 minutes, when the souffle has begun to puff and brown, quickly sprinkle the top with powdered sugar. After a total of 30 to 35 minutes of baking, the top of the souffle should be nicely browned and a toothpick inserted in the middle should come out clean.
