Preheat the oven to 350°F. Grease a 9 × 5-inch metal loaf pan.
In a mixing bowl, cream the butter, 1 cup of granulated sugar, and egg until well combined.
Add the buttermilk, all-purpose flour, and baking soda. Mix until the ingredients form a smooth batter without overmixing.
In a separate bowl, combine the remaining 1/3 cup of sugar with the cinnamon.
Spread about half of the batter, or slightly less, into the prepared loaf pan.
Sprinkle approximately three-quarters of the cinnamon-sugar mixture evenly over the batter.
Carefully spread the remaining batter over the cinnamon layer.
Sprinkle the remaining cinnamon sugar over the top. Draw a knife through the batter to create a swirl.
Bake for about 60 minutes, or until a toothpick inserted into the center comes out clean.
Let the bread cool in the pan for 20 minutes before removing it. Allow it to cool further before slicing.
