Grease a bread loaf pan with cooking fat. Preheat the oven to 175 °C. Mash the bananas with a fork and mix with the butter to a smooth batter. Add eggs, baking powder, milkshake and mix well, ideally with an electric whisk. Add the flour a little at a time and whisk until the batter is smooth. Pour the batter in the greased bread loaf pan and bake in the lower part of the oven for about 50 minutes. Check with a toothpick that the cake is done. Cut along the edge of the pan and place the cake on a platter. Cut into 8 pieces. Serve and enjoy.