Preheat the oven to 350°F (175°C) and grease a 9-inch round cake pan.
In a bowl, whisk together flour, baking powder, baking soda, cinnamon, and salt.
In another bowl, cream the butter and sugar until light and fluffy. Add eggs one at a time, mixing well.
Gradually add the dry ingredients, alternating with buttermilk, starting and ending with the dry ingredients.
In a small bowl, mix the cinnamon and brown sugar for the swirl.
Pour half of the batter into the prepared pan. Sprinkle half of the cinnamon sugar mixture over the batter.
Add the rest of the batter and top with the remaining cinnamon sugar.
Swirl with a knife and bake for 40 minutes or until a toothpick comes out clean.