Preheat oven to 180ºC (160ºC for fan forced) / 350F.
Sift all the dry ingredients – flour, sugar, baking powder, baking soda, and salt into a large bowl. Set aside. Mash bananas and set aside. Combine egg, milk, and oil in a bowl. Set aside.
Make a well in the center of the flour mixture. Add bananas. Pour the egg mixture into the well. Using a large spatula, stir until combined. Do not overmix.
Line a 12-hole muffin tin with paper cases. Divide the muffin mixture evenly among the holes. Place it the oven. Bake for 20-25 mins. When done, let the muffins cool in the pan for few minutes. Then transfer onto a wire rack to cool completely.