Preheat oven to 350°F (175°C). Grease a 9×13-inch baking pan.

In a medium bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.

In a separate large bowl, cream together sugar and butter until light and fluffy.

Add eggs one at a time, then stir in vanilla extract.

Alternately add the flour mixture and buttermilk, beginning and ending with flour. Mix until smooth.

2. Make the Cinnamon Swirl

In a small bowl, combine brown sugar and cinnamon.

Pour melted butter over the mixture and stir until crumbly.

3. Assemble the Cake

Pour half of the cake batter into the prepared pan and spread evenly.

Sprinkle half of the cinnamon swirl mixture over the batter.

Pour the remaining batter on top and gently spread.

Top with the remaining cinnamon mixture.

Use a knife to gently swirl the cinnamon topping into the batter for a marbled effect.

4. Bake the Cake