Preheat your oven to 350 degrees F and lightly grease a 9x13 inch baking dish.
In a large bowl, beat the softened butter and sugar together until the mixture is smooth and creamy.
Add the eggs, vanilla extract, and sour cream to the butter mixture, whisking until well combined.
Fold in the mashed bananas until incorporated.
In a separate bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon.
Gently stir the dry ingredients into the banana mixture until just combined, being careful not to overmix.
Spread the batter evenly into your prepared baking dish.
To make the topping, combine the flour, granulated sugar, brown sugar, and cinnamon in a medium bowl.
Cut the cold, cubed butter into the dry mixture using a fork or your fingers until the topping looks like coarse crumbs.
Scatter the crumb mixture evenly over the top of the cake batter.
Bake for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan before slicing and serving.