Heat the oven to 350°F (175°C). Grease two standard 9 x 5-inch loaf pans and line the bottoms with parchment if desired.

Beat the softened butter and 2 cups granulated sugar until creamy. Beat in the eggs, then mix in the buttermilk.

Whisk the flour, baking soda, 1 teaspoon cinnamon, and salt in a separate bowl. Add the dry ingredients to the wet mixture and stir just until combined.

Combine 2/3 cup sugar and 1 1/2 teaspoons cinnamon. Divide about half of the batter between the prepared pans, sprinkle with cinnamon sugar, add the remaining batter, and finish with the remaining cinnamon sugar.

Bake for 45–50 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs. Cool in the pans for 10–15 minutes, then transfer to a rack to cool further before slicing.