In a medium bowl, combine the brown sugar, white sugar, ground cinnamon, salt, and all-purpose flour. Pour in the melted unsalted butter and mix with a fork until crumbly. Refrigerate while preparing the cake batter.

In a small bowl, stir together the brown sugar and ground cinnamon for the filling and set it aside.

In a large bowl, whisk together the cooled melted unsalted butter, brown sugar, and white granulated sugar until combined. Add in the eggs, sour cream, vanilla extract, and mashed bananas. Blend until smooth.

Gradually add the flour, baking soda, salt, and ground cinnamon to the wet mixture. Stir gently until just combined; avoid overmixing.

Pour half of the batter into a greased and lined 9x9 inch square pan, spread evenly. Sprinkle the filling over the batter, then add the remaining batter on top and smooth it out.

Crumble the chilled topping evenly over the batter.

Preheat the oven to 350°F (175°C) and bake for 35-45 minutes, until golden brown and a toothpick comes out clean. Let it sit for 10 minutes before transferring to a cooling rack.

In a small bowl, whisk together powdered sugar, vanilla extract, and enough milk for a smooth consistency.

Once the cake has cooled, drizzle the glaze over the top and let it set before slicing.