Preheat the oven to 350°F. Lightly grease your loaf pan.
In a small saucepan, brown the butter. Do this by melting the butter, then letting it foam and cook for 3 to 5 minutes, stirring occasionally until flecks of brown form at the bottom of the pan. Transfer it to a bowl or stand mixer and whisk to let it cool slightly. Make sure to get all those good brown bits off the bottom of the pan!
While your butter is browning, make your flax eggs. Combine the ground flax and water, mix, and set aside for at least 5 minutes to gel.
Add the sugar to the butter and mix for about 2 to 3 minutes. Then add in the flax egg, vanilla, and miso. Whisk vigorously into a smooth mixture.
Add the bananas directly into the bowl. Use a fork to mash them into small pieces, but you don't need the bananas completely smooth. Then add to the mixing bowl and mix together.
Add in the flour, baking powder and salt and stir to combine into a thick batter.
Transfer the batter to your lightly greased loaf pan and bake at 350°F for 50 minutes to 1 hour or until a toothpick comes out with just a few crumbs.
When the banana bread is done baking, let it cool for 10 to 15 minutes in the pan and then remove it and place on a wire cooling rack. Let the banana bread fully cool before cutting. Store on the counter, loosely wrapped for 3 days, or in the freezer for up to 3 months.