Preheat the oven to 325°F (160°C). Grease a 9x5-inch (23x13 cm) loaf pan and line with parchment paper, leaving a few inches of overhang.
Add the butter to a small saucepan and melt over medium heat. When the butter has melted and it starts to sputter, reduce the heat to medium-low. Stir gently and constantly until the butter is deep amber in color and emits a nutty aroma, about 4-5 minutes. Immediately transfer the butter to a large bowl and let it cool for 10 minutes.
In a medium bowl, whisk together the flour, cinnamon, baking soda, and salt.
To the bowl of browned butter, add the miso, maple syrup, and brown sugar, and whisk vigorously until smooth (whisking back and forth helps to dissolve the miso better).
Add the mashed bananas, eggs, sour cream, and vanilla extract, and whisk until well combined.
Fold in the flour mixture (and nuts if using) with a silicone spatula until just combined. Transfer the batter to the prepared loaf pan and smooth out the top.
Bake for 65-75 minutes or until a cake tester inserted into the center comes out clean or ideally with a few soft crumbs.
A few minutes before the banana bread is done, make the miso maple glaze by whisking all the ingredients together in a small bowl.
As soon as you remove the banana bread from the oven, gently spread the glaze on top then let it cool in the pan for 10 minutes.
Lift the banana bread out of the pan and cool on a wire rack for at least 45 minutes to 1 hour before slicing.