Preheat your oven to 350°F and generously spray a 12-count muffin pan with non-stick cooking spray, or line with muffin liners.
Whisk together the flour, baking soda, and salt in a medium bowl.
Melt the butter in a large bowl and allow to cool slightly. Whisk in both of the sugars and then whisk in the eggs, one at a time and whisk until just combined after each. Whisk in vanilla extract and sour cream.
In a separate bowl, mash the very ripe bananas until mostly smooth and not too lumpy. Stir the mashed bananas into the wet ingredients.
Add the dry ingredients to the wet ingredients in 3 additions and mix until just coming together after each. After all of the dry ingredients have been added mix until smooth and no more flour pockets remain. Fold ½ cup of the coarsely chopped walnuts into the batter.
Add the batter to the muffin pan filling each up to the top. Sprinkle the remaining ½ cup coarsely chopped walnuts on top.
Bake for 22-24 minutes, until a toothpick inserted into the center comes out clean.