Begin by preheating your oven to 325°F (163°C) and greasing a 9x9 inch baking pan.
Mix together 1/2 cup of flour, 1/2 cup of brown sugar, and a teaspoon of cinnamon. Stir in 1/4 cup of melted butter until the mixture is crumbly, then set it aside.
Take a large bowl and mash the bananas until they reach a smooth consistency.
Add 1/4 cup of melted butter, sour cream, and vanilla essence to the mashed bananas. Mix thoroughly.
In a separate bowl, whisk together 1 cup of flour, baking powder, baking soda, remaining 1/4 cup of brown sugar, and cinnamon.
Gradually add the dry ingredients to the wet mixture, stirring until they are just combined. Take care not to mix excessively to maintain a light texture.
Pour half of the batter into the greased baking pan, spreading it out evenly.
Sprinkle the crumb topping evenly over the batter.
Carefully spoon the remaining batter over the crumbs, covering them entirely.
Optional Touches: If desired, sprinkle chocolate chips or chopped nuts over the top of the batter.
Put the pan in the oven and bake for 40-45 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cake to cool in the pan for 10-15 minutes before slicing and serving. Whether enjoyed warm or at room temperature, it's bound to delight your taste buds.