Preheat oven to 350°F (175°C). Grease two 9x5-inch loaf pans.
In a large bowl, cream together the softened butter and 2 cups of sugar until smooth.
Beat in the eggs one at a time, then mix in the buttermilk.
Add the flour and baking soda, stirring until just combined. The batter will be thick.
In a separate small bowl, mix the 2/3 cup sugar with the cinnamon.
Divide half the batter between the two pans. Sprinkle 3/4 of the cinnamon-sugar mix evenly over the batter.
Add the remaining batter on top. Sprinkle with the remaining cinnamon-sugar.
Swirl gently with a knife or skewer, just enough to marble it—don't overdo it.
Bake for 45-55 minutes, or until a toothpick inserted comes out clean.
Cool in the pans for 10 minutes, then transfer to a wire rack.