A crumbly, sweet pastry with a hot cup of joe is about as perfect a breakfast combination as bacon and eggs.
For filling, in a medium saucepan combine blueberries and the water. Bring to boiling; reduce heat. Simmer, covered, about 5 minutes or until blueberries are tender. In a small bowl stir together 1/4 cup sugar and the cornstarch; stir into blueberry mixture. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 2 minutes more. Set filling aside.
Preheat oven to 350°F. In a medium bowl stir together the 1 1/2 cups flour, the 3/4 cup sugar, the baking powder, and baking soda. Using a pastry blender, cut in the 1/4 cup butter until mixture resembles coarse crumbs. Make a well in the center of flour mixture; set aside.
In a small bowl combine egg, buttermilk, and vanilla. Add egg mixture all at once to flour mixture. Stir just until moistened (batter should be lumpy). Spread half of the batter in an 8x8x2-inch baking pan. Spoon and gently spread filling over batter in pan. Drop the remaining batter in small mounds on top of filling.
In a small bowl stir together the 1/4 cup flour and 1/4 cup sugar. Cut in the 2 tablespoons butter until mixture resembles coarse crumbs. Sprinkle over coffee cake. Bake for 40 to 45 minutes or until golden. Serve warm.