Mash the Bananas: In a medium bowl, thoroughly mash the black-spotted bananas with a fork until they resemble a chunky puree.
Combine Wet Ingredients: In a large mixing bowl, whisk together the melted butter and light brown sugar. Whisk in the room temperature eggs one at a time, followed by the vanilla extract, sour cream, and mashed bananas.
Whisk Dry Ingredients: In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon. This ensures the leavening agent is perfectly dispersed.
Fold to Combine: Pour the dry ingredients over the wet. Using a rubber spatula, gently fold the batter just until the last streak of flour disappears. Do not overmix.
Rest and Bake: Pour the batter into a greased and parchment-lined 9x5-inch loaf pan. Let rest for 10 minutes. Bake at 350°F (175°C) for 55 to 65 minutes until a toothpick inserted into the center comes out clean.
