Preheat the oven to 350°F (177°C). Spray a 9×5-inch or 8×4-inch loaf pan with nonstick spray.

In a small bowl, mix the granulated sugar and ground cinnamon together. Set aside.

In a large bowl, whisk the all-purpose flour, baking soda, and salt together.

In a medium bowl, whisk the granulated sugar, milk, vegetable oil, sour cream, egg, and vanilla extract together. Pour the wet ingredients into the dry ingredients, then whisk just until combined. Avoid over-mixing; the batter should be thick and creamy.

Pour/spread about half of the batter into the prepared loaf pan. Sprinkle almost all of the cinnamon-sugar evenly on top, reserving about 2 Tablespoons for the topping.

Carefully pour/spread the remaining batter evenly on top. Use a spoon to carefully spread it out if it sticks to the cinnamon-sugar filling.

Sprinkle the reserved 2 Tablespoons of cinnamon-sugar on top. Using a knife, make a large swirl down the loaf pan, avoiding over-swirling.

Bake the bread for 50–65 minutes, tenting loosely with foil about halfway through. Poke the center with a toothpick; if it comes out clean, the bread is done.

Allow the bread to cool in the pan on a cooling rack for 1 hour. Then, remove it from the pan and let it finish cooling directly on the rack.

Drizzle with icing, if using. Slice and serve. Store leftover bread at room temperature for up to 3 days or in the refrigerator for up to 1 week.