Combine all topping ingredients and mix with a fork until the texture of coarse crumbs. Set aside.
Cake
Preheat oven to 350 degrees, and grease and flour a 9x13 baking dish.
Combine flour, sugar, baking powder and salt. Cut your butter into your flour mixture until it resembles coarse crumbs.
Crack an egg into a measuring cup and add milk to fill up to 1 cup. Pour into crumb mixture. Add vanilla too and mix until just moistened.
Spread into prepared pan and sprinkle the top with streusel topping. Bake for 25 minutes, or until a toothpick inserted near the center of the cake comes out clean.