Preheat your oven to 350°F (175°C) and prepare the Bundt pan by greasing it with butter or non-stick spray and dusting with flour.

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

Cream together the butter and sugar in a large mixing bowl using an electric mixer for 3-4 minutes until light and fluffy.

Add in the eggs one at a time, then stir in the sour cream and vanilla extract until well combined.

Gradually add the dry mixture to the wet ingredients, alternating with the milk until just combined.

In a small bowl, combine the brown sugar and cinnamon. Pour half the batter into the prepared pan, sprinkle with half of the cinnamon-sugar mixture, then add the remaining batter and top with the rest of the cinnamon-sugar.

Use a butter knife to gently swirl the batter to create a marbled effect.

Bake in the preheated oven for 50 to 60 minutes, or until a toothpick comes out clean.

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