Preheat your oven to 350°F (175°C) and grease two 9x5-inch loaf pans. Set aside.
In a large mixing bowl, cream together the softened butter, 2 cups sugar, and eggs until smooth and fluffy.
Gradually add the buttermilk, then the flour and baking soda. Mix until well combined and a smooth batter forms.
Divide half of the batter evenly between the two prepared loaf pans.
In a small bowl, combine the 2/3 cup sugar with the cinnamon to make the cinnamon sugar mixture. Sprinkle about 3/4 of this mixture evenly over the batter in the pans.
Spread the remaining batter on top, then sprinkle the remaining cinnamon sugar evenly over the top.
Use a knife to gently swirl the batter and cinnamon sugar together to create a marbled effect.
Bake in the preheated oven for 45–50 minutes, or until a toothpick inserted into the center comes out clean.
Allow the loaves to cool in the pans for 10 minutes, then transfer to a wire rack to cool completely before slicing.
Enjoy!
