Roast the Bananas (Optional): Place unpeeled bananas on a foil-lined baking sheet and bake at 350°F for 15 minutes to concentrate the natural sugars for a deeper flavor.
Combine Wet Ingredients: Whisk melted butter and brown sugar. Add room temperature eggs one at a time (to prevent butter from seizing), then stir in vanilla, sour cream, and mashed bananas.
Whisk Dry Ingredients: In a separate bowl, combine flour, baking soda, salt, and cinnamon to ensure leavening agents are evenly dispersed.
Fold the Batter: Gently fold dry ingredients into wet ingredients until just combined. Do not overmix, which creates a tough loaf.
Bake: Pour into a greased 9x5 pan. Bake at 350°F for 55-65 minutes until a toothpick comes out clean.