1. Preheat the oven to 350°F (175°C). Grease or line two 9×5-inch loaf pans.

2. In a large bowl, cream together the butter and sugar until light and fluffy.

3. Add eggs one at a time, then mix in the vanilla extract.

4. In a separate bowl, whisk together flour, baking soda, and salt.

5. Add the dry ingredients to the wet mixture, alternating with the buttermilk, beginning and ending with flour. Mix until just combined.

6. In a small bowl, combine the brown sugar and cinnamon for the swirl.

7. Pour half of the batter into the pans. Sprinkle half of the cinnamon-sugar mixture over the batter.

8. Pour the remaining batter on top, then sprinkle with the remaining cinnamon-sugar. Swirl gently with a knife.

9. Bake for 45 to 55 minutes, or until a toothpick inserted in the center comes out clean.

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