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1
Brush a 5-cup souffle dish with melted butter; dust with brown sugar. Refrigerate until ready to use.
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2
In a medium saucepan, bring 3/4 cup of the milk and the vanilla to the boil over medium heat. Remove and set aside.
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3
In a small bowl, blend the cornstarch and the remaining 1/4 cup milk until smooth. Add 1/4 cup of the sugar, 2 of the yolks and salt and stir until smooth. Gradually whisk this mixture into the warm milk.
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4
Cook over medium-low heat, whisking constantly, until thick, 3-4 minutes. Remove from heat and let cool, about 10 minutes. Beat in remaining 4 yolks, one at a time. Sift flour over mixture in two batches, mixing well.
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5
Cook over medium-low heat, stirring constantly, until smooth. Remove from heat and stir in liqueur and zest. Scrape custard into a bowl and set aside to cool.
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6
Preheat oven to 400 degrees F.
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7
In a large bowl, beat whites until soft peaks form. Slowly add remaining 1/4 cup sugar and beat until firm peaks form. Fold whites into cooled custard.
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8
Transfer to souffle dish and bake for 20 minutes, until puffed and browned on top. Dust top with icing sugar and serve immediately (a souffle waits for no one!).
