Heat the oven to 350°F (175°C). Grease a 9 x 5-inch loaf pan, then line it with parchment if desired, leaving some paper over the long sides to help lift out the baked loaf.

Beat the softened butter and 1 cup of granulated sugar until light and creamy. Add the eggs one at a time, beating after each addition, then mix in the vanilla extract.

In a separate bowl, whisk together the all-purpose flour, baking soda, and salt. Add the dry mixture to the butter mixture in portions, alternating with the buttermilk, and mix only until the ingredients are combined.

Stir the 1/3 cup granulated sugar and ground cinnamon together in a small bowl. Spread approximately half of the batter into the prepared loaf pan, sprinkle with most of the cinnamon sugar, and cover with the remaining batter.

Sprinkle the remaining cinnamon sugar over the top. Run a butter knife through the batter in a few gentle zigzags to create the swirl, taking care not to overmix the layers.

Bake for 50 to 60 minutes, checking early because ovens and loaf pans vary. The bread is ready when a toothpick inserted in the center comes out clean or with a few moist crumbs.

Cool the loaf in the pan for about 10 to 15 minutes, then transfer it to a wire rack. Allow it to cool further before slicing so the crumb can set and the cinnamon layer stays distinct.