Preheat oven to 350°F.
Prepare loaf pans. This recipe makes either 5 mini loaves or 2 regular loaves. I prefer baking mini loaves, because they tend to bake more evenly. And it’s much easier to slice. Spray pans with nonstick spray and set aside.
With electric mixer, cream together the butter, brown sugar, granulated sugar and eggs. Cream about 2 minutes, until fluffy.
Add the vanilla, sour cream, milk and mashed bananas. Mix well.
In a separate bowl, mix the flour, baking soda and salt with a whisk.
Add the flour mixture to the wet ingredients and mix on medium low speed until fully combined, about 1-2 minutes
In a separate bowl, combine the ingredients for the crumble topping. Stir until all is coated with the melted butter. It can be easier to just use your hands for this part.
Fill prepared pans 2/3 full. Sprinkle crumble topping on top of the batter until it reaches the pan level.
Put a baking sheet on bottom rack of oven. This will catch any crumble that might drip during baking.
Bake loaves on center rack for 45-50 minutes for mini loaves and 50-55 minutes for regular size loaves. Insert butter knife in the center of the loaf. If it comes out clean, they are ready to take out. If not, bake another few minutes and check.
Allow the banana bread to cool in the pans for 10 minutes. Turn them out and place on wire rack to cool completely.