Preheat the oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners.
In a medium bowl, whisk together 1 1/2 cups flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, combine mashed bananas, granulated sugar, egg, melted butter, vanilla extract, and sour cream. Mix until smooth.
Add the dry ingredients to the wet ingredients and stir until just combined. Do not overmix.
In a small bowl, prepare the streusel topping by mixing brown sugar, 1/3 cup flour, and cinnamon.
Cut the cold butter into the streusel mixture using a pastry cutter or your fingers until the mixture resembles coarse crumbs.
Spoon the muffin batter evenly into the prepared muffin cups, filling each about 3/4 full.
Sprinkle the streusel topping evenly over each muffin.
Bake for 18 to 22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and let them cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.