Preheat your oven to 350°F (175°C) and grease two 9x5-inch loaf pans.
In a large bowl, cream together the butter, 1 cup of sugar, and eggs until smooth.
Add buttermilk, flour, and baking soda. Stir until just combined—don’t overmix.
In a small bowl, mix the remaining 1 cup of sugar with the cinnamon.
Pour a quarter of the batter into each loaf pan. Sprinkle half the cinnamon-sugar mixture evenly over the batter in both pans.
Add the rest of the batter on top and finish off with the remaining cinnamon sugar.
Use a knife to gently swirl the cinnamon sugar into the batter.
Bake for 45–50 minutes or until a toothpick comes out clean.
Cool in the pan for 20 minutes before transferring to a wire rack.