Preheat the oven to 350°F (175°C). Lightly grease two 9×5-inch loaf pans and optionally line the bottoms with parchment paper.
In a large bowl, cream butter, sugar, eggs, and vanilla using a handheld mixer for 3 minutes until light and fluffy.
Add buttermilk, flour, and baking soda. Fold with a rubber spatula just until combined—do not overmix. A few flour streaks are okay.
In a small bowl, mix cinnamon and sugar thoroughly. Rub between fingers for even distribution.
Divide half the batter evenly into the two loaf pans (approx. 2 cups per pan). Sprinkle ¾ of the cinnamon-sugar mixture evenly over the batter.
Add the remaining batter on top in dollops and spread carefully. Sprinkle remaining cinnamon-sugar mix over the top of each loaf.
Using a butter knife, swirl the batter in each pan by making 8–10 gentle figure-eight motions through the full depth of the batter.
Bake for 45 minutes or until a toothpick inserted in the center comes out clean or with moist crumbs (not wet batter).
Let the loaves cool in the pans for 20 minutes, then remove and place on wire racks. Wait at least 2 hours before slicing for best texture.