Preheat oven to 400°F. Butter six 3/4-cup ramekins; coat with sugar. Spoon 1 teaspoon jam and 4 frozen berries into bottom of each ramekin. Place on baking sheet. Mash lemon peel and 1/2 cup sugar in heavy medium saucepan; whisk in cornstarch, then milk and yolks. Add 2 tablespoons butter. Bring to boil over medium heat, whisking constantly. Boil until thick pudding forms, whisking constantly, about 1 minute. Transfer to large bowl; mix in lemon juice. Season to taste with salt. Using electric mixer, beat egg whites in medium bowl to soft peaks. Gradually beat in 1/4 cup sugar; beat until stiff but not dry. Fold whites into warm lemon pudding. Spoon mixture atop berries; fill to top. Bake until puffed, set, and golden around edges, about 14 minutes. Sift powdered sugar over.