Preheat oven to 375 degrees F. Spray a 9 inch round springform pan with cooking spray.

In a bowl or food processor, combine flour, ½ cup granulated sugar, baking powder and salt. Cut or process in butter until mixture resembles coarse crumbs. Stir or process in the egg and 1 teaspoon of vanilla. Pat lightly into the bottom of the prepared pan.

Sprinkle cornstarch over the blueberries and gently stir to coat (it will disappear as you stir). Spread the blueberries over the crust.

Stir sour cream, ½ cup of sugar, egg yolks and 1 teaspoon of vanilla together in a mixing bowl. Pour this mixture over the blueberries and carefully spread it across the top.

Set the springform pan on a foil lined rimmed cookie sheet** and bake for 60 to 70 minutes in the preheated oven, until the top is lightly browned around edges – it should still look a little loose in the center and will set as it cools and chills. Loosen hot cake from pan with a knife, but do not remove sides quite yet. Cool completely and chill for a few hours. Remove sides of springform pan. Serve each slice with whipped cream or spread whipped cream over entire cake before slicing.