Preheat oven to 350F degrees, oven rack at middle position. Grease and flour a 9 x 5 loaf pan and set aside.
In a medium bowl, mix 1 ¾ cup flour, 1 teaspoon baking powder, ¼ teaspoon baking soda and ¼ teaspoon salt. Set aside.
In the bowl of a stand mixer or using a hand mixer, cream together ½ cup softened butter and 1 cup sugar until creamy, about 3 minutes. Add 1 ½ cups mashed bananas and beat until incorporated. Add 2 eggs, one at a time and beat just until mixed in (don't over mix the eggs in). Add ¼ cup buttermilk, and ½ teaspoon vanilla extract, again just mixing in.
Slowly mix in the flour mixture and beat on medium-low until fully mixed. Pour mixture into loaf pan and then evenly sprinkle a good amount of sugar on top, about 2 Tablespoons. Using the extra banana (doesn't need to be over-ripe, can even be a little under-ripe), slice it into ¼-inch length-wise pieces. If it breaks, no worries. Place the banana slices on top, piecing together if broken.
Bake 45-50 minutes or until a toothpick inserted in the center comes out mostly clean, but kind of shiny. Let the loaf cool on a wire rack at least 20 minutes before turning out (removing). If necessary, as soon as the bread comes out of the oven, take a butter knife and slide it around between the pan and the bread to separate any sugar crust or bread that might stick to the pan.
This delicious bread will probably not last long but can sit out for a day at room temperature. Any leftover after that should be refrigerated. The bread freezes very well but if you plan on freezing it, I wouldn't add the banana slices on top as they'll get kind of mushy in the freezer.