Preparation

• 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 sugar and cinnamon together for the cinnamon swirl.
• 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 and whisk just until combined.
• Pour/spread about half of the batter into the prepared loaf pan. Sprinkle almost all of the cinnamon-sugar on top, reserving about 2 Tablespoons. Carefully add the remaining batter evenly on top.
• Sprinkle the remaining cinnamon-sugar on top and use a knife to make a large swirl down the loaf pan, avoiding over-swirling.
• Bake for 50–65 minutes, tenting with foil halfway through. Check with a toothpick; if it comes out clean, it's done. Cool in the pan for 1 hour, then remove and cool completely 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.