How to Make It
Preheat the oven to 350°. Butter and flour a 10-by-5-inch metal loaf pan. In a bowl, using a fork, mash 4 of the bananas until chunky. In another bowl, whisk the flour, baking soda, baking powder and salt.
Using a stand mixer fitted with the paddle, cream the butter, sugar and miso at medium speed until fluffy, about 5 minutes. At low speed, slowly add the buttermilk, then beat in the eggs 1 at a time until incorporated. Beat in the mashed bananas; the batter will look curdled. Add the dry ingredients and mix until just blended. Scrape into the prepared pan.
Slice the remaining banana lengthwise and arrange the halves on top of the batter side by side, cut side up. Bake for 90 minutes, or until a toothpick inserted in the center comes out clean. Let the bread cool on a rack for 30 minutes before turning out to cool completely.