Preheat your oven to 350°F (175°C). Spray a 9×5 inch loaf pan with nonstick baking spray, then sprinkle it evenly with the tablespoon of cinnamon sugar.

In a large bowl, beat the softened butter and dark brown sugar together until creamy and smooth.

Add vanilla extract, eggs, mashed bananas, sour cream, and molasses to the butter and sugar mixture. Beat thoroughly until well combined.

In a separate bowl, whisk together all-purpose flour, baking soda, salt, cinnamon, ground ginger, nutmeg, and allspice. Gradually stir these dry ingredients into the wet mixture until just combined.

Spoon the batter into the prepared loaf pan, spreading it evenly. Bake for 65 to 70 minutes or until a skewer or toothpick inserted into the center comes out mostly clean.

Remove the bread from the oven and let it cool in the pan for about 10 minutes. Then, carefully invert the loaf onto a wire rack or plate to cool completely.

While the bread cools, stir together powdered sugar and milk in a small bowl until smooth. Pour and spread the glaze evenly over the top of the cooled bread. Sprinkle holiday sprinkles on top if desired.