I don't know about banana, but you can chocolate. For best results- grease and flour your pan, then put in a layer of wax paper then your cake batter. As soon as you can handle the cakes put another layer of wax paper on top, turn out the cake and wrap in several layers of plastic wrap and put directly into the freezer. Do this while it's hot so all of the moisture gets trapped back into the cake. It may swell a bit, but I promise the shape will go back to normal. You cand do this up to a month in advance.