Preheat your oven to 350°F (175°C). Grease and flour a 10-inch Bundt pan.

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon. Set aside.

In a large mixing bowl, cream the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the butter mixture, alternating with the buttermilk, starting and ending with the flour mixture. Mix until just combined.

In a small bowl, combine the brown sugar, ground cinnamon, and melted butter to create the cinnamon swirl mixture.

Pour half of the cake batter into the prepared Bundt pan. Sprinkle half of the cinnamon swirl mixture over the batter. Pour the remaining batter on top and finish with the remaining cinnamon swirl mixture. Use a knife to gently swirl the mixtures together.

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

Allow the cake to cool in the pan for about 15 minutes before inverting it onto a wire rack to cool completely.