Preheat the oven to 200C/400F/Gas 6. Grease a loaf tin with a little sunflower oil.
In a large bowl, mix the mashed banana with the sunflower oil until well combined. Beat in the egg, then stir in the raisins and vanilla extract.
In a separate bowl, mix together the sugar, flour and baking powder.
Stir the dry ingredients into the wet ingredients until well combined.
Pour the cake mixture into the prepared loaf tin and bake in the oven for 30-40 minutes, or until a skewer inserted into the centre of the cake comes out clean. Set aside to cool on a wire rack, then cut into slices and serve.