In a small bowl, mix the flour with the baking soda. In another bowl, using an electric mixer, beat the remaining 4 1/2 tablespoons of butter until creamy. Add the light brown or turbinado sugar and beat until light and fluffy. Add the eggs, 1 at a time, and beat until just incorporated. Beat in the mashed bananas. Using a large spatula, fold the dry ingredients into the batter until incorporated.