Preheat your oven to 350°F (175°C) and grease two 9x5-inch loaf pans.

Cream together butter and granulated sugar until light and fluffy, about 3-4 minutes.

Incorporate eggs one at a time, then add buttermilk and blend until mixed.

Whisk together flour and baking soda, then gradually add to the wet ingredients without overmixing.

Mix brown sugar and ground cinnamon together for the cinnamon swirl mixture.

Layer half the batter in loaf pans, sprinkle half the cinnamon mixture, then add the remaining batter and remaining cinnamon mixture.

Gently swirl through the layers with a butter knife.

Bake for 45-50 minutes until a toothpick comes out clean.

Let cool in pans for 10 minutes before transferring to a wire rack.