Mix the flour, water, baking powder, egg and salt and let sit for about 20 minutes. This is your batter. Dip the banana halves in the batter and deep fry in vegetable oil at 325 degrees F for about 5 minutes or until the batter is golden brown. Remove the bananas from the oil and allow to drain. Meanwhile mix the cream, brown sugar and butter together in a sauce pan and cook over medium heat until the sugar has dissoved. Add the rum and be sure the sauce doesn't boil. Place some vanilla ice cream on a plate. Place a banana over the ice cream and pour a generous amount of rum sauce over the dessert. Serve while the banana is still hot.