Preheat an oven to 175°C.
Take an 11cm x 21cm loaf pan, line it with baking paper, and lightly grease it with spray oil.
Add the cream butter, castor sugar and brown sugar into a large mixing bowl, and blend them together until the resulting mixture is smooth. Add the eggs and vanilla. Use an electric mixer to beat the mixture for 3 minutes to form a smooth batter.
Sift the all-purpose flour, baking powder, baking soda, and cinnamon into a large bowl.
Add the batter mixture to the flour mixture, and mix them together until they combine. Add the mashed bananas, mix them together for 30 seconds, then add the milk. Mix all the ingredients on low speed until they combine.
Pour the resulting batter into the pre-prepared cake or bread pan, then bake it for 35–45 minutes (or until a skewer inserted into centre comes out clean).
Remove the bread from the oven, and allow it to cool in the pan for 5minutes. Turn the bread onto a wire rack to cool for 1 hour (or until completely cool). Cut the bread into slices to serve.