Preheat your oven to 350°F. Grease a loaf pan measuring 9x5 inches with butter or a nonstick spray.
In a medium bowl, mix together the flour, baking soda, and salt. Set this aside.
In a large bowl, beat together the softened butter and granulated sugar using an electric mixer on medium speed until light and fluffy, about 2 minutes. Add the eggs one at a time, blending in the vanilla extract.
Keep the mixer on low speed, alternating between adding a third of the flour mixture and half of the buttermilk. Repeat this process, making sure to finish with the flour mixture. Mix just until combined.
In a small bowl, combine the light brown sugar with the ground cinnamon.
Pour half of the batter into the prepared loaf pan and smooth it out. Evenly sprinkle half of the cinnamon sugar mixture over the top. Pour the rest of the batter over that and smooth it out again. Top with the remaining cinnamon sugar.
Take a butter knife and gently swirl through the batter to create a marbled effect, being careful not to overmix.
Bake in the oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean. Allow it to cool in the pan for around 10 minutes before transferring to a wire rack to cool completely.