Preheat your oven to 350°F (175°C) and grease your bundt pan with unsalted butter or non-stick spray.

In a small bowl, combine ground cinnamon and brown sugar, then set aside.

In a large mixing bowl, cream together unsalted butter and granulated sugar until light and fluffy (about 4-5 minutes).

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

In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually add to the butter mixture, alternating with the milk.

Pour half of the batter into the bundt pan, sprinkle with the cinnamon sugar mixture, then add remaining batter on top and swirl gently with a knife.

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

Let the cake cool in the pan for about 10-12 minutes, then invert onto a wire rack to cool completely.