Preheat oven to 350°F. Prepare a 9x5-inch loaf pan by lining with a parchment paper sling or spraying with baking spray.
In a large bowl, combine the sugar, eggs, sour cream, milk, oil, and vanilla extract and whisk together.
1 cup (200g) granulated sugar, 2 large large eggs, 1/2 cup sour cream, 1/2 cup milk, 1/3 cup vegetable oil, 2 teaspoons pure vanilla extract
Add the flour, baking powder, baking soda, and salt. Stir together until combined. Set aside.
2 cups (250g) all-purpose flour, 2 teaspoons baking powder, 1 teaspoon baking soda, 1/2 teaspoon salt
In a small bowl, mix the cinnamon and brown sugar together.
1/2 cup brown sugar, 1 Tablespoon ground cinnamon
Add about 1/2 of the batter to the prepared loaf pan. Sprinkle 3/4 of the cinnamon sugar mixture evenly over the top of the batter, then use a butter knife to swirl it in a few times.
Add the remaining batter on top of the cinnamon sugar layer, then sprinkle with the remaining cinnamon sugar mixture.
Bake for 50 to 65 minutes until a skewer inserted in the center of the bread comes out clean.