In a large bowl mix together the butter, sugars, eggs, and vanilla, make sure there are no traces of egg left behind.
Mix Wet Ingredients
Whisk together the flour, baking soda, baking powder, and salt.
Mix Dry Ingredients
Add mashed banana and sour cream then mix to combine to wet ingredients.
Add Bananas & Sour Cream
Gently fold in the dry ingredients and chopped roasted walnuts.
Mix until just combined, do not ovemrix.
Pour into the prepared pan. Sprinkle optional brown sugar and walnuts on top. Bake and let cool on a wire rack.