Preheat oven to 180C/160C fan-forced. Spray a 7cm-deep, 10 x 20cm (base measurement) loaf pan with oil. Line base and sides with baking paper, extending 3cm above rim.
Sift flour into a large bowl. Stir in sugar. Make a well in the centre. Add butter, egg, vanilla, milk and banana. Stir until just combined.
Pour into prepared pan. Bake for 30-35 minutes or until a skewer inserted into centre comes out clean. Set aside in pan for 10 minutes to cool slightly. Serve warm or transfer to wire rack to cool completely.