Blueberry Buttermilk Coffeecake

1/2 cup butter
1 1/3 cups white sugar
2 eggs
1 teaspoon vanilla extract
2 cups buttermilk
4 cups all-purpose flour
3 teaspoons baking powder
1 teaspoon baking soda
1 teaspoon salt
2 cups blueberries

2/3 cup all-purpose flour
2/3 cup white sugar
6 tablespoons butter, softened

Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan. Sift together 4 cups flour, baking powder, baking soda and salt. Set aside.

In a large bowl, cream together 1/2 cup butter and 1 1/3 cup sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the buttermilk, mixing just until incorporated. Sir in blueberries. Pour batter into prepared pan. In a small bowl, combine 2/3 cup flour and 2/3 cup sugar. cut in 6 tablespoons butter until mixture resembles coarse crumbs. Sprinkle over top of batter.

Bake in the preheated oven for 45 to 60 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.