Preheat your oven to 350°F (175°C). Grease two 9x5-inch loaf pans.

In a large mixing bowl, whisk together the flour, 1 ½ cups sugar, baking powder, baking soda, salt, and 1 tablespoon cinnamon until well combined.

In another bowl, mix the buttermilk, vegetable oil, eggs, and vanilla extract until smooth.

Gradually add the wet ingredients to the dry ingredients, stirring until just combined. Do not overmix.

In a small bowl, combine ½ cup sugar and 2 tablespoons cinnamon for the swirl mixture.

Pour half of the batter into the prepared loaf pans. Sprinkle half of the cinnamon-sugar mixture over the batter.

Pour the remaining batter on top and finish with the rest of the cinnamon-sugar mixture.

Use a knife to gently swirl the batter and cinnamon mixture together.

Drizzle the melted butter over the top of the loaves.

Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.

Let the loaves cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.