Preheat the oven to 350˚ with the rack in the middle.
Grease a 10" cast iron skillet (or any heatproof skillet) with butter or vegetable oil.
Melt the butter and let it cool to room temperature. Mix the cooled butter with the brown sugar in a large bowl. Mix in the egg, mashed banana and vanilla.
Whisk the flour, baking soda, salt and cinnamon together in another bowl.
Add the banana mixture into the flour mixture, combining well. Do not over mix.
Transfer the batter into the prepared skillet, smooth with top with the back of a spoon or spatula. Top with the crumble, pressing the crumble gently down into the batter with your fingers. Bake for 30-35 minutes or until a wooden skewer comes out virtually crumb free. Let cool completely before slicing and serving.
Combine the sugars, cinnamon and salt in a large bowl. Add the flour and stir with a rubber spatula until mostly combine. Using clean fingers, mix until everything is well combined.