Preheat your oven to 350°F (175°C) and grease a 9x5 inch loaf pan.
Whisk the 2 cups flour, 1 cup sugar, baking soda, and salt together in a large bowl.
Mix the egg, buttermilk, vegetable oil, and vanilla extract in a separate bowl until smooth.
Fold the wet ingredients gently into the dry ingredients until just combined, being careful not to overmix.
Combine the remaining 1/3 cup sugar and cinnamon in a small bowl.
Pour half the batter into the pan, sprinkle with half the cinnamon sugar, add the remaining batter, and top with the rest of the sugar mixture.
Bake for 45 to 50 minutes until a toothpick inserted in the center comes out clean. Cool in the pan for 20 minutes.