Preheat oven to 350℉ Lightly oil two 9x5 inch oaf pan.
Stir together mashed bananas and buttermilk; set aside.
Cream brown sugar and margarine together until light. Beat in egg. Add banana mixture; beat well.
Sift together flour, baking powder, baking soda and salt; add all at once to liquid ingredients. Stir until well blended.
Stir in nuts
Pour into prepared pan.
Bake for 50 - 55 minutes or until toothpick inserted in center comes out clean. Cool 5 minutes in pan. Remove from pan and complete cooling on a wire rack before slicing.