Pre-heat your oven to 170°c (150°c for fan-assisted oven or Gas Mark 3) and line a muffin tray with twelve cupcake cases.
In a large bowl, mash the bananas before adding the butter and sugar. Mix well until combined.
In a separate bowl (or mug) whisk the egg, milk and vanilla before adding to the mixture.
Sift the flour and baking powder into the mixture and gently fold to form a batter.
Using an ice cream scoop, transfer the batter to your prepared cupcake cases, filling each to about 2/3 full.
Bake in your pre-heated oven for 15-20 minutes, until risen, golden and a skewer inserted comes out clean.
Once cool enough to touch, transfer to a wire rack to cool completely.