Preheat your oven to 350°F (175°C) and grease and flour a bundt pan to prevent sticking.

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

In a large bowl, beat 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 sour cream and vanilla extract until smooth.

Gradually incorporate the dry ingredients and milk alternately into the butter mixture, mixing gently until just combined.

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

Pour half of the batter into the prepared bundt pan, sprinkle half of the cinnamon-sugar mixture on top, and then pour the remaining batter over.

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

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 15 minutes, then carefully invert onto a wire rack to cool completely.

For the glaze, whisk together the powdered sugar, milk, and vanilla extract until smooth. Drizzle over the cooled cake before serving.