Preheat oven to 350 degres F. Spray an 8½ by 4½-inch loaf pan with non-stick cooking spray; set aside.
In a large bowl, whisk together the flour, baking soda and salt.
Place the bananas in a microwave-safe bowl; cover with plastic wrap and cut several steam vents in plastic with a paring knife. Microwave on high power until the bananas are soft and have released liquid, about 5 minutes. Transfer the bananas to a fine-mesh strainer placed over a medium bowl and allow to drain, stirring occasionally, or 15 minutes (there should be ½ to ¾ cup liquid).
Transfer the liquid to a small saucepan and cook over medium-high heat until reduced to ¼ cup, about 5 minutes. Remove the pan from the heat, stir the reduced liquid into the bananas, and mash with a potato masher until fairly smooth. Whisk in the butter, eggs, brown sugar and vanilla extract.
Pour the banana mixture into the flour mixture and stir until just combined, with some streaks of flour remaining. Gently fold in the walnuts, if using. Scrape the batter into the prepared pan. Sprinkle the granulated sugar evenly over the top of the loaf.
Bake until a toothpick inserted into the center of the loaf comes out clean, 55 to 75 minutes. Cool the bread in the pan on a wire rack for 15 minutes, then remove the loaf from the pan and continue to cool on a wire rack. Serve warm or at room temperature. The bread can be wrapped tightly in plastic wrap and stored at room temperature for up to 3 days. It can also be frozen (the wrapped loaf should then be placed in an resealable freezer bag or double-wrapped with foil) for up to 2 months.
It's important that the bananas are not just speckled, but are full-on browning or even black so they can release those delicious juices.
You can use thawed frozen bananas; if you do so, you do not need to microwave them, simply go to placing them over a fine-mesh strainer (they will release a lot of liquid naturally).
If you use a 9x5-inch loaf pan, start checking for doneness 5 minutes sooner than the recipe states.
You can substitute other chopped nuts for the walnuts; use what you love (or omit them completely)!
This banana bread can be frozen by wrapping in plastic wrap, then foil, and placing it in a freezer bag; freeze for up to 3 months.