Preheat oven to 350°F (175°C). Lightly spray a 9x5 inch (23x13 cm) light-colored loaf pan with non-stick spray and line it with crinkled white parchment paper.

In a large bowl, whisk together the mashed overripe bananas, melted butter, granulated sugar, eggs, sour cream, and vanilla extract until completely smooth.

In a separate bowl, whisk the flour, baking soda, and salt. Gently fold this into the wet banana mixture using a spatula until just combined. Do not overmix.

In a small bowl, mix the dark brown sugar, ground cinnamon, and melted butter with a fork until it resembles wet sand.

Pour exactly half of the batter into the prepared loaf pan. Sprinkle the wet cinnamon sugar mixture evenly over this layer. Spoon the remaining batter over the top, using a knife to gently marble the layers.

Bake in the preheated oven for 50 to 60 minutes, or until a skewer inserted into the center comes out clean and a golden-brown cracked crust forms. Cool in the pan for 15 minutes before transferring to a wire rack to cool completely.

Once the bread is fully cooled to room temperature, whisk together the powdered sugar, heavy cream, and vanilla extract until thick and completely smooth.

Generously drizzle the bright white vanilla icing glaze over the top of the cooled loaf, letting it drip down the sides and pool on the parchment paper. Slice and serve.