1. Preheat oven to 375°. Butter 6 ramekins, 1 1/4- to 1 1/2-cup size. Set in a rimmed baking pan.
2. Melt butter in a medium saucepan over medium-high heat. Whisk in flour, then half-and-half and 1/3 cup sugar. Cook, whisking, until mixture boils; boil 30 seconds more. Remove from heat and whisk in zest and juice until smooth. Whisk in egg yolks.
3. In a large bowl of a mixer, beat egg whites on high speed until frothy. Add cream of tartar and beat until soft peaks form. Gradually add remaining 1/3 cup sugar, beating until whites hold straight peaks when beaters are lifted.
4. Fold 1/4 of whites into yolk mixture, then fold all of yolk mixture into rest of whites. Spoon into ramekins (they'll be nearly full).
5. Bake until soufflés no longer jiggle when gently shaken, 15 to 18 minutes. Serve immediately.
Note: Nutritional analysis is per serving.