To make the sauce: In a medium saucepan, stir together sugar, cornstarch and water. Bring to a boil, reduce heat to medium and cook, stirring occasionally, for 5 minutes or until sugar dissolves. Stir in blueberries and cook, stirring occasionally for 10 minutes. Reduce heat to low, add butter and stir until melted. Stir in balsamic vinegar. Remove from heat and set aside.
To make the soufflés: Preheat oven to 350 degrees. Butter 6 individual (6- to 8-ounce) soufflé dishes or 6 large ramekins.
Using an electric mixer or food processor, blend together the eggs, sour cream, orange juice, butter, 2 1/2 tablespoons sugar and baking powder. Add flour and blend until smooth. Pour half the batter (about 1/4 cup each) into the prepared baking dishes.
Beat together the cream cheese, cottage cheese, egg yolks, vanilla and remaining 1/2 tablespoon sugar. Drop by spoonfuls over the top of the batter in the baking dishes. Gently drizzle remaining batter over the top (about 1/4 cup per dish). Put ramekins on a large baking sheet and bake uncovered, for 30 to 35 minutes or until puffed, set in the center and golden brown. Serve with warm sauce spooned over the top. Garnish with additional fresh blueberries.
MAKE AHEAD: Blueberry Balsamic Sauce can be made up to two days ahead, covered and refrigerated. Can also be frozen for up to 6 months. Soufflés can be assembled but not baked up to 12 hours ahead, covered and refrigerated. Bring to room temperature before baking.
NOTE: Blueberry Balsamic Vinegar can be purchased at specialty food stores or online. If you can’t find it, substitute white balsamic vinegar and add a few more blueberries (to taste).
ONE-DISH VARIATION: Make the soufflé in one dish. Butter an 8- by 8-inch baking dish. Pour half the batter (about 1 1/2 cups). Top with cheese mixture as described, then drizzle remaining batter over the top. Bake, uncovered, 40 to 45 minutes or until puffed and set in the center and golden brown. Serve with Blueberry Balsamic Sauce on the side.