Using a 2-quart saucepan, melt butter over medium heat. Whisk in flour and salt until blended; gradually stir in milk. Cook, stirring often, until mixture boils and thickens; boil 1 minute. Remove from heat.
Stir 1/4 cup granulated sugar into milk mixture. Whisk in egg yolks until well blended. Transfer to medium bowl, cool slightly, (about 30 minutes). Stir in liqueur and peel.
Preheat oven to 375 degrees F. Grease ten 6-ounce ramekins; sprinkle with 3 tablespoons granulated sugar.
In a large bowl, with mixer at high speed, beat egg whites and remaining 2 tablespoons sugar until stiff peaks form when beaters are lifted. With rubber spatula, gently fold one-third of whites into milk mixture, fold milk mixture gently back into remaining whites.
Spoon mixture into prepared ramekins. Bake just until set, 30 minutes. When souffles are done, sprinkle with confectioners sugar (if desired). Serve immediately.