Brush a 5-cup souffle dish with melted butter; dust with brown sugar. Refrigerate until ready to use.
In a medium saucepan, bring ¾ cup of the milk and the vanilla to the boil over medium heat. Remove and set aside.
In a small bowl, blend the cornstarch and the remaining ¼ cup milk until smooth. Add ¼ cup of the sugar, 2 of the yolks and salt and stir until smooth. Gradually whisk this mixture into the warm milk.
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.
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.
Preheat oven to 400 degrees F.
In a large bowl, beat whites until soft peaks form. Slowly add remaining ¼ cup sugar and beat until firm peaks form. Fold whites into cooled custard.
Transfer to soufflé dish and bake for 20 minutes, until puffed and browned on top. Dust top with icing sugar and serve immediately (a soufflé waits for no one!).