Preparation

• Preheat the oven to 350°F (177°C). Spray a 9×5-inch or 8×4-inch loaf pan with nonstick spray.
• For the Cinnamon Swirl: In a small bowl, mix the sugar and cinnamon together. Set aside.
• Make the Bread: 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.
• 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). Carefully pour/spread the remaining batter evenly on top. Sprinkle the remaining 2 Tablespoons of cinnamon-sugar on top. Using a knife, make a large swirl down the loaf pan.
• Bake the bread for 50–65 minutes, tenting loosely with foil about halfway through. If it comes out clean when poked with a toothpick, it's done. Cool in the pan for 1 hour, then finish cooling on a rack.
• Drizzle with icing, if using. Slice and serve. Store leftovers at room temperature for up to 3 days or in the refrigerator for up to 1 week.