Grease a 1 kg (2 lb) loaf tin. Beat the butter and sugar together in a large bowl with a hand-held electric whisk until pale and fluffy.
Whisk in the eggs, mashed bananas, buttermilk and vanilla extract until well combined.
Sift over the flour, bicarbonate of soda, baking powder and salt and gently fold in with a large metal spoon, then stir in the chopped pecan nuts.
Spoon the mixture into the prepared tin and arrange the pecan halves down the centre.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 50–60 minutes or until risen and golden brown and a skewer inserted into the centre comes out clean. Cover the top of the loaf with foil if it becomes too brown.
Leave the loaf to cool in the tin for a few minutes, then turn out on to a wire rack to cool completely before serving.