Preheat oven to 350 degrees and grease an 8x8 pan.

Combine flour, baking powder, baking soda, and salt; set aside.

In a large bowl, mash bananas. Add sugar, brown sugar, sour cream, oil, egg, and vanilla; whisk until smooth.

Pour dry ingredients into wet ingredients and gently fold until combined; do not over mix.

To make crumble topping, combine all topping ingredients until mixture is crumbly.

To make cinnamon sugar filling, combine filling ingredients.

Pour about 2/3 of the batter into the pan and gently spread out evenly. Sprinkle filling mixture on top, then cover with the remaining batter. Top with crumble topping by gently dropping pieces onto the batter; do not flatten or pat down.

Bake for 30-40 minutes or until an inserted knife comes out clean. Let cool.

For icing, whisk powdered sugar with 2 tablespoons of milk; if thinner consistency is desired, add more milk. Drizzle over cooled cake and cut.