Preheat oven to 350 degrees with the rack in the top third. Spray 1 muffin tray with non stick cooking spray, set aside.

In a large mixing bowl, whisk together flour, baking soda and salt. Set aside.

In a microwave safe medium size mixing bowl, add the 5 peeled bananas, cover with plastic wrap and poke a few holes to ventilate. Microwave on high for 5 minutes. Bananas will be soft and have released about 1/2 cup to 3/4 cup of liquid. Transfer bananas to fine-mesh strainer placed over medium bowl and allow to drain, stirring occasionally, 15 minutes.

In a small sauce pan, cook the banana liquid over medium heat until it has reduced to 1/4 cup, about 5 minutes. Combine the bananas, banana liquid, butter, eggs, light brown sugar and vanilla extract. Stir until most lumps are removed.

Pour the wet ingredients into the dry ingredients and stir until just combined. Add in walnuts and gently fold into the batter. Fill each muffin cup 3/4 full. Will make exactly 12 muffins. Bake for 40 minutes, rotating the tray half way thru. Cool muffins in tray for 10 minutes, then remove from tray and allow to cool for another 5 minutes.

Will keep in an airtight container for 5 days.