Preheat the 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 sour cream and milk. Begin and end with the dry ingredients. Mix until just combined.

In a small bowl, combine the brown sugar and ground cinnamon for the swirl.

Pour half of the batter into the prepared Bundt pan. 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 mixture through the batter.

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 10 minutes before inverting onto a wire rack to cool completely.