Preheat oven and butter pan:
Heat oven to 350°F (175°C). Butter a 9x5-inch loaf pan.Mix wet ingredients:
In a large bowl, stir together mashed bananas, 1/2 cup granulated sugar, brown sugar, melted butter, eggs, and vanilla until smooth.Fold in dry ingredients:
Scatter baking soda and salt over the banana mixture. Add flour and fold gently just until incorporated. Do not overmix.Make cinnamon-sugar mixture:
In a small bowl, combine 1/4 cup granulated sugar with cinnamon. Reserve.Layer half the batter:
Transfer half the batter to the prepared loaf pan. Evenly sprinkle half the cinnamon-sugar mixture on top.Add remaining batter:
Add the remaining batter over the cinnamon layer. Top with the rest of the cinnamon-sugar.Swirl to marble:
With a knife or skewer, lightly swirl the batter to create a marble pattern. Avoid overmixing.Bake until done:
Bake for 50-60 minutes, until a toothpick inserted in the center comes out clean.Cool in pan then rack:
Let cool in the pan for 10-15 minutes, then turn out onto a wire rack to cool fully.Whisk glaze:
In a small bowl, whisk powdered sugar with 1-2 tablespoons milk until smooth and pourable. Add more milk or sugar as needed to adjust consistency.Drizzle glaze over bread:
Once the bread is completely cool, drizzle the glaze over the top.