Preheat the oven to 350℉. Butter and flour a 9x5 loaf pan.

In a medium bowl, add the butter, flour, dark brown sugar, granulated sugar, salt and cinnamon. Use your hands to combine the ingredients, scrunching them up together to form large crumbs (some will be smaller, some larger). Place in the refrigerator while you make the batter for the banana bread.

In a large bowl, cream together the butter and sugar.

Add the mashed bananas and mix to combine. You can mash the bananas with a beater in a large 4-cup measuring cup to ensure that you have precisely 2 cups.

Add the eggs and vanilla and combine.

Add the sour cream, baking soda, and salt and combine.

Lastly, add the flour and mix in with a spoon until just combined. Incorporating the flour with a spoon helps to prevent overmixing, which helps prevent a dense loaf.

Pour the batter in the prepared baking dish. Top with the streusel crumb topping.

For a ceramic 9x5 loaf pan bake at 350℉ for 1 hour 5 minutes up to 1 hour and 10 minutes. For a metal 9x5 loaf pan bake at 350℉ for 50 minutes up to 1 hour and 5 minutes. If the streusel topping starts to burn, tent the pan with aluminum foil midway through baking.