Preheat oven to 350°F and lightly grease a large oven-safe ramekin.
Make the crumb topping by combining flour, brown sugar, and cinnamon in a small bowl. Stir in melted butter until large crumbs form. Set aside. 3. In a medium bowl, whisk together flour, brown sugar, granulated sugar, baking powder, cinnamon, and salt.
Add mashed banana, oil, Greek yogurt, milk, and vanilla. Stir until just combined.
Pour batter into the prepared ramekin.
Sprinkle crumb topping evenly over the batter.
Bake for 20–25 minutes, or until the center is set and a toothpick comes out with a few moist crumbs.
Allow to cool for 10 minutes.
Whisk together powdered sugar, milk, and vanilla until smooth. Drizzle over the warm cake.