01 - Preheat oven to 350°F. Grease a 9×5-inch loaf pan or line with parchment paper.
02 - In a large bowl, whisk together all-purpose flour, granulated sugar, baking powder, baking soda, and salt.
03 - In a separate bowl, whisk buttermilk, melted unsalted butter, eggs, and vanilla extract until fully combined and smooth.
04 - Gradually fold the wet mixture into the dry ingredients, stirring gently just until incorporated. Do not overmix.
05 - In a small bowl, combine brown sugar and ground cinnamon thoroughly.
06 - Pour half of batter into the prepared pan, sprinkle with half the cinnamon-sugar mixture, then repeat layers. Gently swirl a knife through the batter to marble.
07 - Bake for 50 to 60 minutes, or until a toothpick inserted into the center comes out clean.
08 - Allow the bread to cool in pan for 10 minutes, then turn out onto a wire rack to cool completely.
09 - Brush the top of the warm bread with melted butter. Mix granulated sugar and ground cinnamon, then sprinkle evenly over the surface.
10 - Slice into portions and serve at room temperature for best texture.