Preheat the oven to 325 degrees F. Generously grease two 8 1/2 x 4 1/2-inch loaf pans and set aside.

In a large bowl, cream 1/3 cup softened butter and 1 1/4 cups white sugar until light and fluffy.

Add the 2 eggs to the butter-sugar mixture and mix until fully incorporated.

Stir in 1 cup sour cream, 4 large very ripe mashed bananas, and 1 tsp vanilla extract until combined.

In a separate bowl, whisk together 2 1/2 cups all-purpose flour (or white whole wheat), 1 tsp cinnamon, 2 tsp baking soda, and 1/2 tsp salt.

Add the dry ingredients to the wet ingredients and stir gently until just combined; do not overmix.

Fold in 1 cup chopped walnuts until evenly distributed.

Divide the batter evenly between the two prepared loaf pans and smooth the tops.

Bake in the preheated oven for 1 hour, or until a toothpick inserted into the center comes out clean.

Remove the pans from the oven and let the loaves cool in the pans for about 10 minutes, then transfer the loaves to a wire rack to cool completely before slicing.