Heat overn to 350? and grease two 9" cake pans. Cream butter or margarine until soft and add sugar a little at a time, beating until light and creamy. Add the eggs one at a time, beating thoroughly. Sift together flour, baking powder, salt, and soda. Combine the mashed bananas, vanilla and milk, mixing well after each addition. To creamed mixture add dry ingredients alternately with banana mixture beating well after each addition. Pour batter into pans and bake 30 minutes oruntil toothpick inserted at center comes out clean. Cool completely. Arrange sliced bananas between layers. This cake is great plain but you may wish to ice it with your favorite frosting or just pour cream filling over top. NOTE: To make cake flour put two tbl cornstarch in a 1 cup measuring cup and add flour to reach 1 cup, sift 3 times then use. ANOTHER NOTE: Sour milk can easily be made by just adding 1 tsp white vinegar to 1 cup of regular milk.