Preheat oven to 325F. Grease a 9x5x3 loaf pan or line with parchment paper.
- Sift the flour together with baking soda and baking powder.
- Add salt.
- In another bowl mix melted butter with sugar.
- Add eggs and buttermilk.
- Stir in flour mixture, then mashed bananas.
- Fold in walnuts and orange zest.
- Transfer into a prepared pan.
- Bake for 1 hour and 10 minutes.
- Cool on a wire rack.