Preheat oven to 350°F. Lightly grease two 9×5-inch loaf pans.
In a large bowl, cream together butter, 2 cups sugar, eggs, and vanilla using a handheld mixer until light and fluffy.
Using a rubber spatula, add buttermilk, flour, and baking soda just until combined. Don't overmix.
In a small bowl, combine ⅔ cup sugar and cinnamon for topping.
Divide half the batter between prepared pans. Sprinkle three-fourths of cinnamon mixture over batter.
Spread remaining batter evenly over cinnamon layer. Top with remaining cinnamon mixture.
Using a butter knife, cut a few gentle swirl marks through the batter.
Bake 45 minutes or until toothpick inserted near center comes out clean.
Cool in pans 20 minutes, then remove to wire racks to cool completely.
