Prep the Oven & Pan: Preheat your oven to 175°C (350°F). Grease a small 6-inch cake pan or line it with parchment paper.
Whisk the Wet Ingredients: In a mixing bowl, vigorously whisk the 130g mashed banana, egg, butter, and milk until uniform.
Add the Dry Ingredients: Sift in the flour, baking powder, baking soda, cinnamon, and salt. Stir gently with a spatula just until the flour disappears—do not overmix.
Bake: Pour the batter into your prepared pan. Bake for 20 to 25 minutes. A toothpick inserted into the centre should come out clean.
Cool: Let it cool in the pan for 5 minutes, then transfer to a wire rack to cool completely