Preheat your oven to 350°F (175°C) and grease an 8x8 inch pan.
In a bowl, mix together flour, baking powder, baking soda, and salt; set aside.
In a large bowl, mash the bananas. Add the sugars, sour cream, oil, egg, and vanilla, whisking until smooth.
Gently fold the dry ingredients into the wet mixture until just combined.
For the crumble topping, mix all topping ingredients until crumbly.
Combine the cinnamon sugar filling ingredients in a small bowl.
Pour about two-thirds of the batter into the prepared pan, spreading it evenly. Sprinkle the filling mixture on top, then cover with the remaining batter. Add the crumble topping without flattening it.
Bake for 30-40 minutes, or until a knife inserted comes out clean. Allow to cool.
For the icing, whisk powdered sugar with 2 tablespoons of milk, adding more milk for a thinner consistency. Drizzle over the cooled cake and slice!