Preheat your oven to 350°F. Spray a standard 9 x 5 inch loaf pan with non stick cooking spray and set aside. You can line the pan with parchment paper if you'd like for easy removal, however it's optional.
In a large bowl, whisk the flour, baking soda, salt, and cinnamon together.
In another bowl, with a hand mixer using the paddle or whisk attachment, beat the butter and brown sugar together on medium-high speed until smooth.
On medium speed, add the eggs one at a time, continue beating after each has been added. Add in the sour cream, mashed bananas, and vanilla extract on medium speed until combined.
Slowly beat the dry ingredients into the wet ingredients on low speed until there are no remaining flour pockets.
Pour the batter into the prepared loaf baking pan. Whisk the flour, brown sugar, and cinnamon together then cut in the cold butter with a fork until crumbles form. Sprinkle on top of the batter. *See note regarding extra crumble
Bake for 50-60 ish minutes. You can create an aluminum foil tent on top of the loaf pan and loosely cover half way through to help prevent the top from getting too brown.
When a toothpick inserted in the center of the loaf comes out clean, the bread is done. Remove from the oven and allow the bread to cool completely. Enjoy!