Mix up butter, eggs, sugar and vanilla in a bowl and let sit. Then, sift baking powder, soda, flour and salt in a bowl. Add these to the first mixture. Add in the bananas and sour milk (or buttermilk). Beat well and add to prepared cake pans (9-in. round or 9×13) lined with wax paper or greased/floured. Bake for 17-20 min. at 350F (if layered) or 25-40 min. if you are baking a sheet cake.