Begin by preheating your oven to 350°F (175°C). While the oven is heating, grease your Bundt pan with unsalted butter and dust it lightly with flour.

In a large mixing bowl, whisk together 2 ½ cups of all-purpose flour, 1 tablespoon of baking powder, and ½ teaspoon of salt until combined.

In another bowl, use an electric mixer to beat 1 cup of softened unsalted butter and 1 ½ cups of granulated sugar together until light and fluffy.

With the mixer on low speed, add 3 large eggs one at a time, then mix in 1 cup of sour cream and 1 tablespoon of vanilla extract.

Gradually add the dry ingredients to the wet mixture while alternating with ½ cup of milk, mixing just until combined.

In a small bowl, combine 1 cup of brown sugar with 1 tablespoon of ground cinnamon. Spread half of the cake batter into the Bundt pan, sprinkle the cinnamon swirl mixture over this layer, and top with the remaining batter.

Using a butter knife, gently swirl through the batter to create a marbled effect.

Place the Bundt pan in the preheated oven and bake for 50 to 60 minutes, until a toothpick comes out clean.

Let the cake cool in the pan for about 15 minutes before inverting onto a wire rack.

For the glaze, whisk together 1 cup of powdered sugar with 2 tablespoons of milk and ½ teaspoon of vanilla extract until smooth, then drizzle over the cooled cake.