Using an electric mixer, cream together 1/2 cup butter and 1 cup sugar. Beat together the eggs and milk, and stir into the creamed mixture. Mix together 2 cups flour and 2 t baking powder, and add it to the butter mixture. Add the vanilla and stir just until combined. Better to undermix than overmix; I'm sorry to harp on it, but I want you to have a moist, tender cake.
Pour the batter into a greased 9" x 13" baking pan. If you're feeling generous, or are watching your weight, you can use two smaller pans and give one of the cakes away :)
In a separate bowl, mix together the remaining 2 cups flour, 3 t baking powder, and 1 t salt; cut in the 2 T shortening. Stir in the cinnamon and remaining 1 cup sugar, then cut in the remaining 1/2 cup butter until the crumbs are a size that makes you happy. Sprinkle it over the batter and bake at 350 degrees for about 30 minutes, or until a toothpick comes out clean.
Let the cake cool, then dust it with powdered sugar.
Bisquick can be used in the topping instead of the flour, baking powder, salt, and shortening combo. I keep a simple pantry with basic ingredients, so I make mine the hard way ;)