Preheat the oven to 350°F (175°C). Line two standard muffin tins with paper liners and spray lightly with nonstick spray.
In a large bowl, cream together the softened butter (1/2 cup), granulated sugar (1 cup), and light brown sugar (1/3 cup) with an electric mixer until light and fluffy.
Add the eggs (2), vanilla (1 tsp), and sour cream (1 cup) to the creamed mixture and mix until just combined.
In a separate bowl, whisk together the all-purpose flour (2 1/4 cups), baking powder (2 tsp), baking soda (1 tsp), and salt (1 tsp).
Gradually add the dry ingredients to the wet ingredients and stir with a wooden spoon or spatula until just combined; avoid overmixing.
Fold in the mashed banana (1 1/4 cups, about 3 bananas) and, if using, 1 cup chocolate chips until evenly distributed.
Fill each muffin liner about two-thirds full. If using extra chocolate chips, sprinkle the remaining 1/2 cup on top of the batter in the liners.
Bake for 12–15 minutes, or until the muffins are lightly browned and a toothpick inserted into the center comes out clean.
Transfer the muffins to a wire rack to cool before serving.