Preheat your oven to 325F and grease and line an 8×8 baking pan, allowing the paper to hang over the sides for easy removal.
Start by making the crumble – Combine the flour, cinnamon, and sugars in a bowl. Then mix in the melted butter. The mixture should resemble wet sand. Place the bowl in the refrigerator to chill while the rest of the cake is made.
For the banana cake, whisk together the flour, baking soda, and salt in a medium bowl. Set aside.
In a large mixing bowl, whisk together the sugars and butter.
Once fully combined, whisk in the sour cream, egg, vanilla, and mashed banana.
Gently mix in the dry ingredients using a rubber spatula.
Spread half of the batter into the pan. Combine the cinnamon sugar mixture and sprinkle an even layer on top.
Cover with the remaining batter and gently spread it even.
Remove the crumble from the refrigerator. It should now be cold and firm. Crumble it on top in an even layer.
Bake for 40-45 minutes or until a toothpick in the center comes out clean with a few moist crumbs.
Allow the cake to cool for about 30-40 minutes.
While it cools, whisk together the powdered sugar and milk to make the icing. It should make a glue-like consistency. If it’s too thick, add ½ tablespoon of milk at a time until it turns smooth.
Drizzle the icing on top of the slightly warm cake and enjoy!