Preheat oven to 350 degrees F (175 degrees C). Grease 2 loaf pans.
In small bowl, mix flour, baking powder, baking soda, and salt. Set a side.
In another bowl, cream butter, sugar, and brown sugar until well blended. Add eggs and vanilla. Mix well. Mix in flour mixture.
Stir in mashed bananas, buttermilk, and nuts (optional). Spread in prepared loaf pan.
Bake at 375 degrees F (175 degrees C) for 40-55 minutes, or until a toothpick inserted into the center of the loaf comes out clean. Cool loaf in the pan for 10 minutes before removing to a wire rack to cool completely.