Preheat oven to 350°F. Grease a 9×5-inch loaf pan and set aside.

In a small bowl, combine 1/3 cup sugar with cinnamon to make the cinnamon sugar mixture.

In a large bowl, whisk melted butter and egg, then add buttermilk and vanilla, mixing well.

Add flour, baking soda, and the cinnamon sugar mixture into the wet ingredients, stirring until just combined.

Pour half the batter into the prepared loaf pan, then sprinkle generously with cinnamon sugar.

Add the remaining batter on top and sprinkle remaining cinnamon sugar evenly over the top.

Bake for 50 minutes, or until a toothpick inserted in the center comes out clean.

Cool in pan for 10 minutes, then transfer to a wire rack to cool completely.