Preheat the oven to 180°C (160°C fan) and grease a 23cm x 13cm loaf tin. Line it with baking parchment.
In a medium bowl, whisk together 190g plain flour, 1 tsp bicarbonate of soda and ½ tsp salt.
In a large bowl, whisk together 115g melted unsalted butter, 100g caster sugar and 55g light brown sugar until smooth.
Beat in 2 large eggs and 1 tsp vanilla extract until the mixture is creamy.
Fold in 3 ripe bananas, mashed, until just combined – it’s fine to leave a few small lumps.
Add the dry ingredients to the wet ingredients and gently stir until just combined; a few streaks of flour should remain.
Fold in 150g dark chocolate chips, reserving a handful to sprinkle on top.
Spoon the batter into the prepared tin and bake for 50–60 minutes, or until a skewer inserted into the centre comes out clean or with a few moist crumbs and the top is golden brown.
Leave the loaf to cool in the tin for 10–15 minutes, then transfer it to a wire rack to cool completely before slicing.