Heat oven to 375° F. Grease one 16-ounce or two 8-ounce ramekins with 1 tablespoon of butter. Coat with 1 tablespoon sugar, then tap out the excess. Set aside.
Melt the remaining butter in a large saucepan over medium heat and whisk in the flour. Blend in the milk and cook, stirring constantly, until the sauce boils, 1 minute. Remove from heat. Blend in the liqueur and zest. Whisk in the egg yolks one at a time. Set aside. (Recipe can be made to this point up to 1 day ahead. Cover and refrigerate.)
Beat the egg whites, salt, cream of tartar, and remaining 2 tablespoons of sugar until stiff (but not dry) peaks form. Pour the sauce into the whites; gently fold together. Spoon into ramekins. Bake until puffed and browned, 40 minutes for a 16-ounce soufflé, 20 minutes for two smaller ones. Serve immediately with Custard Sauce.