In the bowl of a stand mixer with a paddle attachment, beat together the butter and 2 cups sugar until light and fluffy.
Add the eggs and vanilla extract. Combine until mixed well, scraping down the sides as needed.
Add the flour, buttermilk, and baking soda slowly as the mixer spins so as not to spread flour everywhere.
Pour 1/4 of the batter into 2 bread pans.
Mix the 1/3 cup of sugar with the 2 teaspoons of cinnamon.
Sprinkle 1/4 of the mixture on top of the batter in the bread pans.
Divide the remaining batter between the two bread pans and top with remaining cinnamon-sugar mixture.
Use a toothpick or knife to create swirls on the top.
Bake for 50-60 minutes, or until a toothpick comes out clean when inserted into the center.
