Preheat the oven to 180C/160C fan. Grease and line a 450g loaf/1lb tin with baking parchment (or a wider tin for a thinner, flatter cake - this works well with the gluten-free buckwheat option).
Mix the flour, baking powder, cinnamon and sultanas.
In another bowl, mix the butter, vanilla essence, egg, milk and mashed bananas with a fork.
Pour the banana mixture into the flour mixture and combine thoroughly with a wooden spoon.
Pour the cake mixture into the prepared tin and bake for 40 mins or until a skewer inserted in the middle comes out clean. Remove from the oven, allow to cool in the tin for 10 mins then turn out.
Drizzle with agave syrup/honey if using.