Preheat oven to 400°F. Coat six 3/4-cup soufflé dishes with butter and sugar. Bring milk to simmer in medium saucepan over medium heat. Whisk 1/4 cup sugar, cornstarch, and yolks in bowl to blend. Gradually whisk in hot milk. Return mixture to saucepan; bring to simmer, whisking constantly. Remove from heat. Whisk in lemon juice and peel. Cool.
Using electric mixer, beat egg whites to soft peaks. Gradually beat in remaining 1/2 cup sugar. Continue beating until stiff but not dry. Fold 1/4 of whites into lemon mixture. Fold in remaining whites. Spoon mixture into prepared dishes, dividing equally. Bake until puffed and brown on top, about 18 minutes. Serve immediately.
