Preheat an oven to 175°c/155°c fan (350°f/310°f).
Next, brown the butter. To do this, chop the butter into evenly sized pieces then place into a medium saucepan. Place over a medium heat & allow to melt, stirring regularly with a spatula. Keep cooking, until the butter starts to foam, smells nutty & you can see that the milk solids in the bottom of the pan have turned golden brown.
Once the brown butter is cooked, transfer it to a bowl then leave to cool slightly. The butter will take around 15 minutes to cool.
In the meantime, grease a 2 lb loaf tin with butter then line the bottom & two longer sides with a piece of baking parchment, leaving a couple of centimetres overhanging the tin's edges (this makes it easier to take the cake out of the tin once it is cooked!).
Next, peel the bananas, add into a large mixing bowl then mash with a whisk (or fork!) until almost completely smooth. A few small lumps are fine!
Add the brown butter & brown sugar to the bananas then whisk to combine. Then add in the sour cream, eggs & vanilla extract then whisk again until thoroughly combined.
Next, sift the flour, bicarbonate of soda, baking powder, cinnamon & ginger into a separate bowl then add in the table salt & whisk well, to combine.This ensures that the raising agents are evenly distributed throughout the batter. Add the dry ingredients into the banana mix then use a spatula to gently fold in until almost fully incorporated (a few streaks of flour are fine at this point!). Add in the chopped pecans then fold again, to mix them through the batter.It's important not to over mix the batter, as this will make the banana bread dense! Any streaks of flour will get mixed in when the pecans are added. Once the batter's been mixed, it needs to go into the oven as soon as possible, as the bicarbonate of soda will have started to react. Transfer the banana bread batter into the lined loaf tin then pipe a thin line of softened butter straight down the middle of the cake. This controls how the banana bread cracks on top as it bakes. Bake the banana bread in the preheated oven for 50 minutes - 1 hour, until a skewer inserted into the centre comes out clean (a few moist crumbs are fine).