In a mixing bowl, beat the bananas and butter until creamy. Add the eggs and beat well. Combine flour, baking powder, baking soda and cinnamon; add to banana mixture alternately with water, beating well after each addition. Stir in nuts. Spoon into a greased 23 cm square baking pan. Bake at 180ºC for 30 minutes or until a toothpick comes out clean. Cool.