In a mixing bowl, combine ½ cup all-purpose flour, ⅓ cup brown sugar, ⅓ cup granulated sugar, 1 teaspoon ground cinnamon, and a pinch of salt. Pour in ¼ cup melted unsalted butter, and mix with a fork until the mixture resembles clumpy sand. Chill the streusel in the refrigerator.

In a small bowl, mix together ⅓ cup brown sugar and 1 tablespoon ground cinnamon until well combined. Set aside for later.

In a large bowl, whisk together ½ cup melted unsalted butter and ½ cup brown sugar with ½ cup white granulated sugar until smooth. Add in 2 large eggs, ½ cup sour cream, 1 teaspoon pure vanilla extract, and 2 medium overripe bananas (mashed) and mix until creamy.

Gently fold in 1 ½ cups all-purpose flour, 1 teaspoon baking soda, and a pinch of salt into the wet mixture until no dry flour spots remain.

Grease a 9x9-inch baking pan and spread half of the batter evenly across the bottom. Spoon the cinnamon filling over the batter, followed by the remaining batter. Swirl layers with a knife and sprinkle the chilled streusel on top.

Preheat your oven to 350°F (175°C) and bake for 35–45 minutes, or until a toothpick comes out clean. The top should be golden brown.

Once baked, let it cool in the pan for about 10-15 minutes. Prepare a glaze by whisking together 1 cup powdered sugar and 2-3 tablespoons of milk until smooth. Drizzle over the cooled cake.