Preheat oven to 350°F (175°C). Grease two loaf pans and line with parchment paper.
In a large mixing bowl, cream softened butter, 2 cups sugar, and eggs until fluffy.
Add buttermilk, flour, and baking soda. Stir until batter is smooth.
Spread ¼ of the batter into each loaf pan.
Sprinkle ¾ of the cinnamon sugar mixture over the batter in both pans.
Top with the remaining batter.
Use a knife to gently swirl the cinnamon sugar through the batter.
Bake for 45–50 minutes or until a toothpick comes out clean.
Let loaves cool in the pans for 20 minutes, then transfer to a wire rack. Slice and enjoy.
