Preheat oven to 350 degrees. Prepare an 8 1/2- by 4 1/2-inch loaf pan by greasing it or lining it with parchment paper (allowing an overhang on sides). Beat egg, mashed banana and melted butter in a large bowl until thoroughly blended. Stir in milk.
Combine flour, sugar, baking powder, baking soda and salt. Add to banana mixture and stir until just moistened. Scrape batter into prepared pan. (You can slice some banana rounds on top for presentation, if you like.)
Bake for 55-65 minutes, until toothpick inserted into the centre comes out clean.