Heat your oven to 350°F (175°C). Grease two standard loaf pans (8x4-inch) with butter or non-stick spray.
In a large bowl, cream together softened butter, sugar, and eggs until fluffy and pale.
Stir in buttermilk until fully combined.
Add flour and baking soda, mixing until just incorporated. Do not overmix
In a small bowl, mix together ⅔ cup sugar and cinnamon.
Pour half of the batter (about ¼ into each loaf pan).
Sprinkle ¾ of the cinnamon-sugar mix evenly over the batter.
Add the remaining batter on top, then finish with the rest of the cinnamon-sugar mix.
Use a butter knife to gently swirl the cinnamon mixture into the batter for a marbled effect.
Bake for 45–50 minutes or until a toothpick inserted in the center comes out clean.
Cool in the pans for 20 minutes, then transfer loaves to a wire rack to finish cooling.
