Preheat and Prepare Pans
Set your oven to 350°F (175°C). Grease two 9×5-inch loaf pans.
Make the Batter
In a large bowl, beat the softened butter, sugar, and eggs together until the mixture is fluffy and pale. In another bowl, combine the flour and baking soda. Alternately add the flour mixture and buttermilk to the butter mixture, starting and finishing with the flour. Mix just until blended—avoid overmixing.
Mix Cinnamon Sugar
In a small bowl, stir together the sugar and cinnamon.
Layer the Batter and Cinnamon Sugar
Divide about half of the batter evenly between the two prepared pans. Sprinkle roughly ¾ of the cinnamon sugar mixture over the batter in each pan. Spread the remaining batter on top, then sprinkle the rest of the cinnamon sugar over the second layer. Use a knife or skewer to gently swirl the cinnamon sugar into the batter.
Bake
Place the pans in the oven and bake for 45–50 minutes, or until a toothpick inserted in the center comes out clean.
Cool and Serve
Allow the loaves to cool in the pans for 20 minutes, then carefully transfer them to a wire rack to cool completely.
