Remove upper rack from oven and set remaining rack to center position. Preheat oven to 350 degrees.
Grease a 5-inch by 9-inch loaf pan and set aside.
In a large bowl, whisk together flour, baking soda, baking powder, salt, and 1 teaspoon cinnamon. Set aside.
In a medium bowl, whisk together mashed bananas, oil, ½ cup brown sugar, vanilla extract, eggs, and buttermilk to combine.
Gently fold the banana mixture into the dry mixture and stir until just combined and no dry flour is visible. Scrape into prepared loaf pan.
Slice remaining banana in half lengthwise and gently lay on top of the batter, cut-side down. In a small bowl, combine a half teaspoon cinnamon, 2 tablespoons brown sugar, and flaky salt, then sprinkle evenly over batter and banana.
Place pan on prepared rack toward rear of oven and bake for 30 minutes, then rotate pan 180 degrees. Continue baking for another 30 to 35 minutes, or until inserted skewer comes out clean and edges of the loaf pull away from the pan.