Preheat the oven to 350°F (177°C). Spray a 9×5-inch or 8.5×4.5-inch loaf pan with nonstick spray.

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

In a large bowl, whisk the flour, baking soda, and salt together. In a medium bowl, whisk the sugar, milk, oil, sour cream, egg, and vanilla extract. 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 (reserve about 2 Tablespoons for topping). It’s a very thick layer of cinnamon-sugar. Carefully pour/spread the remaining batter evenly on top. The top layer of batter will try to stick to the cinnamon-sugar filling, making spreading a little more difficult, but use a spoon to carefully spread it out as best you can. Sprinkle the remaining 2 Tablespoons of cinnamon-sugar on top. Using a knife, make a large swirl down the loaf pan, as shown in the video tutorial. Avoid over-swirling, which will mix the layers together too much.

Bake the bread for 50–65 minutes, tenting loosely with foil about halfway through to prevent the top from over-browning. Bread baked in an 8×4-inch pan takes closer to 65 minutes. Poke the center of the bread with a toothpick. If it comes out clean, the bread is done. Remove from the oven and allow the bread to cool in the pan set on a cooling rack for 1 hour. After that, remove the bread from the pan and let it finish cooling directly on the rack.

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