Grate 2 1/2 teaspoons peel and ream 1/3 cup plus 1/2 cup juice from lemons; set aside.
Lightly butter 6 ramekins, 1 1/4- to 1 1/2-cup size. Evenly space in a 10- by 15-inch rimmed baking pan.
Melt 4 tablespoons butter in a 2- to 3-quart pan over medium-high heat. Whisk in flour, then half-and-half and 1/3 cup granulated sugar. Whisk constantly until mixture comes to a boil; boil 30 seconds. Remove from heat. Whisk in 1/3 cup lemon juice, yolks, and 2 teaspoons lemon peel.
In the bowl of a mixer, beat egg whites until foamy. Add cream of tartar and beat until soft peaks form. Beat in 1/3 cup granulated sugar, 1 tablespoon at a time, just until whites hold straight, glossy peaks when beaters are lifted.
Stir about 1 cup egg whites into yolk mixture, then gently fold yolk mixture into whites until no streaks remain. Spoon mixture evenly into ramekins (they'll be full).
For a creamy texture, bake in a 375° oven until softly set (when gently shaken, soufflés should ripple in center only), 12 to 13 minutes. For a drier texture, bake 20 minutes.
Meanwhile, in a 1- to 2-quart pan, combine cornstarch and 6 tablespoons granulated sugar. Stir in remaining 1/2 cup lemon juice. Stir over medium-high heat until bubbling, 3 to 5 minutes. Stir in remaining 1/2 teaspoon lemon peel.
Sift powdered sugar over soufflés. At the table, spoon a hole in the center of each soufflé and pour in sauce.