Preheat the oven to 425ºF. For standard-size muffins, line a 12-cup muffin tin with paper liners or spray with nonstick cooking spray. For jumbo muffins, line a 6-cup jumbo muffin tin with paper liners or spray with nonstick cooking spray. Set aside
In a large bowl, add the vegetable oil, brown sugar, milk and sour cream. Whisk until well combined, making sure to break up any clumps of brown sugar. Add the eggs and whisk again.
⅓ cup vegetable oil, 1 cup dark brown sugar, ½ cup whole milk, ½ cup full-fat sour cream, 2 large eggs
Add in the mashed banana and stir with a wooden spoon or spatula until well combined.
1 cup mashed overripe bananas
Gently fold in the flour, baking powder, salt, and cinnamon until no dry spots of flour are left.
2 ½ cups all-purpose flour, 1 tablespoon baking powder, 1 teaspoon salt, 1 teaspoon cinnamon
Fill each cup of muffin tin almost to the top with muffin batter. For jumbo muffins, this will make about 6 muffins. For standard-size muffins, this will make about 12 muffins.
Bake for 7 minutes at 425ºF, then lower the oven temperature down to 350ºF for another 18-20 minutes for jumbo muffins or 15 minutes for standard-size muffins, or until a toothpick inserted into the center muffin comes out with just a few crumbs on it. Allow to cool in muffin tins for 10 minutes, then transfer to a wire rack to cool completely.