Preheat your oven to 350°F (175°C) and grab your bundt pan. Generously grease and flour the pan, ensuring to cover all intricate details to prevent sticking later.

In a medium bowl, whisk together the all-purpose flour, baking powder, and salt until well combined.

In a large mixing bowl, beat the softened unsalted butter and granulated sugar together using an electric mixer on medium speed for 3 to 4 minutes until light and fluffy.

Add the large eggs one at a time to the butter-sugar mixture, mixing well after each addition. Incorporate the sour cream and vanilla extract until everything is just combined.

Gradually add the flour mixture to the wet batter, alternating with the milk, starting and ending with the dry ingredients, mixing gently until just combined.

In a small bowl, stir together the brown sugar and ground cinnamon until well blended. Pour half of the cake batter into your prepared bundt pan, then evenly sprinkle the cinnamon-sugar mixture on top. Carefully add the remaining batter.

Using a butter knife, gently swirl through the batter in a figure-eight motion to create a beautiful marbled effect.

Place the bundt pan in the preheated oven and bake for 50 to 60 minutes. Check for doneness by inserting a toothpick into the thickest part of the cake.

Once baked, let the Cinnamon Swirl Bundt Cake cool in the pan for about 15 minutes. Then, gently invert the cake onto a wire rack to cool completely.