Preheat oven to 375°F/190°C. Grease and flour a 9"x 5" loaf pan and/or line with parchment paper.
Large a bowl, mash bananas with a fork until creamy but not over mashed. You still want to have a few chunks of banana here and there.
In a mixing bowl cream together butter and sugar until light and fluffy, about 2-3 minutes. Add eggs, sour cream and vanilla. Blend until smooth, scraping down sides of bowl. Stir in mashed bananas.
In another bowl combine the all purpose flour, baking soda, baking powder, cinnamon (if using) and salt. Stir gradually into the creamed mixture in 2 or 3 parts. Before all traces of flour disappear, stir in nuts if using. Mix until just combined without over mixing.
Spread batter into prepared loaf pan and place in the oven. Reduce the heat to 350℉/177℃ and bake for 55-60 minutes or until a cake tester inserted in the center comes out clean. Do not over bake.
Cool banana bread in pan for 20 minutes before removing onto cooling rack. Cool completely before serving.