Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan. Make the streusel topping: In a medium bowl, combine 1/4 cup flour, 2/3 cup sugar and 1 teaspoon cinnamon. Cut in butter until mixture resembles coarse crumbs. Set aside.
In a large bowl, combine 2 cups flour, 3/4 cup sugar, baking powder and salt. Cut in butter until mixture resembles coarse crumbs. Crack an egg into a measuring cup and then fill add milk to make 1 cup. Stir in vanilla. Pour into crumb mixture and mix just until moistened. Spread into prepared pan. Sprinkle top with streusel.
Bake in the preheated oven for 25 to 30 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.