Banana and walnut bread

125g unsalted butter
1 cup caster sugar
2 eggs
3-4 very ripe bananas, mashed - 1.5 cups
2 cups plain flour
1 tsp baking powder
1.5 tsp baking soda
1 cup sour cream
1 cup walnuts, chopped
butter, to serve

1. Preheat oven to 170C. Line a 22x12cm medium loaf pan with baking paper. Beat butter and sugar in an electric mixer on high speed until mix is light and creamy. Beat in eggs. Fold in banana using a large metal spoon.
2. Sift in flour, baking powder and baking soda into mixture. Add sour cream and nuts, then gently fold in with metal spoon. Set aside for 2 hours so baking soda can lighten the density of the mixture and it darkens slightly.
3. Spoon into prepared tin and bake in centre of oven for 1 hour 10 mins or u ntil cooked when tested with a skewer. Slice and serve with butter.