Preheat oven to 325 degrees. In a large mixing bowl, cream together the butter, sugar, brown sugar, eggs, and vanilla. In a separate bowl, sift together the flour, baking soda, baking powder, cinnamon and salt. Add dry ingredients alternately with buttermilk and mashed bananas into the creamed mixture, starting and ending with dry ingredients. Stir in nuts. Pour batter evenly into two well-greased and floured 8x4-inch loaf pans. Bake 45 minutes or until a toothpick inserted into the center comes out clean. Cool in pan for 10 minutes, and then remove to a wire rack to cool completely.