Preheat the oven to 180°C (160°C fan/Gas Mark 4) and grease a 2 lb loaf tin, then line it with baking parchment leaving an overhang on the long sides.
Peel and place 4 medium bananas into a bowl, then mash with a fork until mostly smooth with a few small lumps and set aside.
In a large bowl, cream together 150 g softened unsalted butter and 150 g caster sugar until light, pale and fluffy using an electric whisk or wooden spoon.
Beat in 2 large eggs one at a time, mixing well after each addition. If the mixture looks curdled, add a spoonful of flour and continue beating.
Sift 225 g self-raising flour, 1 teaspoon bicarbonate of soda and a pinch of salt into a separate bowl to combine evenly.
Add the dry ingredients, the mashed bananas and 1 teaspoon vanilla extract to the butter mixture, then gently fold together with a spatula until no streaks of flour remain.
Transfer the batter to the prepared loaf tin, smooth the top, and bake for 50–60 minutes or until golden and a skewer inserted into the centre comes out clean. If it browns too quickly, cover loosely with foil for the last 10–15 minutes.
Allow the loaf to cool in the tin for 10–15 minutes, then lift out using the parchment overhang and transfer to a wire rack to cool completely.