Place an oven rack in the center position and preheat oven to 350°F (175°C). Grease two 8x4-inch loaf pans generously with non-stick spray or line with parchment paper, leaving overhang on the long sides.
In a medium bowl, whisk together 2 cups all-purpose flour, 1 teaspoon baking soda, and ½ teaspoon salt for about 30 seconds until thoroughly combined. Set aside.
In a large bowl, beat ½ cup softened unsalted butter and 1 cup granulated sugar with an electric mixer on medium speed for 2 to 3 minutes until pale, light, and fluffy. Scrape down the sides as needed.
Add 2 large eggs one at a time, beating well after each. Pour in 1 cup buttermilk and 1 teaspoon vanilla extract, and mix until well combined. The batter may look slightly curdled, which is normal.
Gradually add the dry ingredient mixture to the wet ingredients, about a third at a time, mixing on low speed or stirring gently with a spatula. Stop as soon as no dry flour streaks remain; do not overmix.
In a small bowl, stir together ⅓ cup granulated sugar and 1 tablespoon ground cinnamon with a fork until evenly mixed and no clumps remain.
Divide half the batter evenly between the two prepared loaf pans and spread evenly. Sprinkle half the cinnamon-sugar mixture over the batter in each pan. Top with remaining batter, then sprinkle remaining cinnamon-sugar over the top of each loaf.
Use a butter knife or skewer to gently swirl through the batter in a figure-eight pattern a few times, pulling cinnamon sugar down through the layers. Do not over-swirl. Tap each pan lightly on the counter once to settle the batter.
Bake in the preheated oven for 45 to 50 minutes, checking at 45 minutes. Bread is done when a toothpick inserted into the center comes out clean or with just a few moist crumbs, and tops are golden brown. If browning too quickly, loosely cover with foil for the last 10 minutes.
Let bread cool in pans for about 10 minutes, then use parchment overhang or run a knife around edges to lift loaves out. Transfer to a cooling rack and let cool completely before slicing.