In a large saucepan cook blueberries and water over medium heat for 5 minutes. Combine sugar and cornstarch in a small bowl. Add lemon juice, sugar and cornstarch to fruit; bring to a boil. Cook for 2 minutes or until thickened. Set aside to cool.

In a large bowl combine flour, sugar, baking powder, and baking soda. Add butter until mixture becomes coarse crumbs. Stir in eggs, sour cream, and vanilla.

Spread half the batter into a greased 13x9 inch pan. Add blueberry filling then remaining batter.

Sprinkle topping over cake.

Bake at 350 for 40-45 minutes. Combine confectioners sugar, milk, and vanilla extract in small bowl. Drizzle over warm cake.