Preheat oven to 180 degrees C. Grease and flour a 23x33cm cake tin.
In a medium bowl combine 1/4 cup flour, 2/3 cup sugar and 1 teaspoon cinnamon. Rub 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. Rub in butter until mixture resembles coarse crumbs. Crack an egg into a measuring cup and then add milk to make 1 cup. Stir in vanilla. Pour into crumb mixture and mix until just moistened. Spread into prepared tin. 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.