Grease and flour an 8 X 11 inch dish and set aside.
Mix the softened butter and sugar, until smooth and creamy.
Add banana's, eggs, vanilla and sour cream, and blend until smooth.
In a separate bowl, blend the flour, baking powder and salt, and add to the mixing bowl.
Mix until completely incorporated.
In a large bowl, make the topping by mixing the flour, brown sugar add the cinnamon, and stir well.
Pour the melted butter over the mixture and stir until crumbly.
Pour the batter into the dish and sprinkle the crunch topping on top of the batter.
Bake at 350 for 45 minutes, or until a toothpick inserted in the center comes out clean.
Remove from the oven and place on a wire rack to cool.