Preheat oven to 170°c - 340°f. Line a loaf pan with spray oil & parchment paper.
Place the salted butter, brown sugar & unrefined caster sugar into a stand mixer bowl, attach the paddle & mix on medium speed for 4 minutes until pale, creamy & smooth. Add in the eggs one at a time, continue mixing for 1 minute until smooth. Stop the stand mixer.
Add in the plain (all-purpose) flour, baking powder & bicarb soda, mix over low speed until just incorporated, scrape down the bowl with a spatula.
Add in the roughly chopped banana, vanilla extract & full-fat milk. Mix over low speed for 10 seconds, increase the speed to medium & mix for 1 minute or until fully incorporated.
Transfer the batter into the prepared loaf pan and bake in the preheated oven for 45 to 50 minutes or until an inserted skewer comes out clean. Remove & let cool for 10 minutes. Slice, portion & dig in.