Preheat oven to 375 degrees.

In a large bowl, combine the flour, sugar, baking powder and salt.

In a small bowl, whisk the butter, eggs, milk, and vanilla together. Stir into dry ingredients just until moistened (do not overmix); set aside. in a small bowl, combine topping ingredients.

Spread half of batter into a greased 13x9 inch baking dish. Sprinkle with half of the topping. Carefully spread remaining batter over top; sprinkle with remaining topping. Do not worry if the batter will not cover the streusel topping completely - the flavors all blend together nicely while cooking, and it all turns out delicious.

Bake at 375 degrees for 25-30 minutes or until a toothpick comes out clean. Serve warm.