Preheat oven to 325 degrees Fahrenheit (165 degrees Celsius) and grease a 10-inch tube pan or two 9x5 inch loaf pans.

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

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

In a large stand mixer bowl, cream the butter and granulated sugar on medium-high speed until light and fluffy, approximately 5 minutes.

Add eggs one at a time, beating well after each addition and scraping down the sides of the bowl.

Incorporate the vanilla extract into the butter mixture.

Reduce mixer speed to low and add the flour mixture in three parts, alternating with the sour cream in two parts, beginning and ending with the flour.

Pour one-third of the batter into the prepared pan and sprinkle with half of the cinnamon sugar mixture.

Repeat the layering with another third of the batter and the remaining cinnamon sugar, then top with the final third of the batter.

Gently swirl a knife through the layers to create a marble effect.

Bake for 60 to 75 minutes, or until a wooden skewer inserted into the center comes out clean.

Cool in the pan for 15 minutes before transferring to a wire rack to cool completely.