Preheat oven to 350° F.
Grease 9×5 inch loaf pan: brush with softened butter and sprinkle an even layer of flour around the pan. Set aside.
In a medium skillet add the mashed bananas, butter, brown sugar, and miso. Cook and whisk banana mixture over medium heat until butter melts and mixture begins to simmer. Turn the heat down to low and let simmer for ~3-5 minutes, whisking occasionally.
5 medium overripe bananas, 1/2 cup brown sugar, packed, 5 Tbsp. unsalted butter, 1/4 cup white miso paste
Remove from heat and let slightly cool for about 10 minutes. It does not have to cool all the way, but just so it is not super hot.
In a large bowl, combine the eggs, oil, buttermilk and vanilla extract. Add the cooled down banana mixture to this and whisk together.
2 large eggs, 1/3 cup vegetable oil, 1/4 cup buttermilk, 1 tsp. vanilla extract
In a separate mixing bowl, combine the flour, sugar, baking soda, baking powder and salt. Add the dry ingredients to the wet ingredients and gently fold until combined. Do not over mix the batter.
1 3/4 cup all purpose flour, 1 cup sugar, 1 tsp. baking soda, 1/2 tsp. baking powder, 1/2 tsp. salt
Pour this mixture into the prepared pan. Bake for ~45-50 minutes, or until a toothpick inserted comes out clean.
Allow bread to cool for 30 minutes before turning out to cool completely.