1. Preheat your oven to 350°F and grease a 9×13-inch baking pan with butter or cooking spray, or line it with parchment paper.

2. In a small bowl, combine the ½ cup flour, brown sugar, and 1 teaspoon cinnamon for the crumb topping. Add the cold butter pieces and use a fork or your fingers to work the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces. Set aside.

3. In a large mixing bowl, mash the very ripe bananas with a fork until mostly smooth. Add the melted butter, granulated sugar, sour cream, eggs, and 2 teaspoons vanilla extract, whisking together vigorously for about 30 seconds until smooth and slightly lighter in color.

4. In a medium bowl, whisk together the 2 cups flour, baking soda, baking powder, salt, and ½ teaspoon cinnamon until evenly distributed.

5. Add the dry ingredients to the wet ingredients all at once and use a wooden spoon or rubber spatula to gently fold everything together. Mix just until you don’t see any more dry flour streaks. The batter should look thick and slightly lumpy.

6. Pour the batter into your prepared baking pan and spread it evenly. Sprinkle the crumb topping evenly over the entire surface of the batter, using all of it.

7. Bake for 35-40 minutes, until the top is golden brown, the crumb topping is crispy and caramelized, and a toothpick inserted into the center comes out with just a few moist crumbs.

8. Let the coffee cake cool in the pan for 15-20 minutes. While cooling, whisk together the powdered sugar, milk, and ½ teaspoon vanilla extract until smooth and pourable. Drizzle the glaze over the warm cake in a back-and-forth pattern. Let it set for about 10 minutes before slicing.