Cream 1 cup of unsalted butter with 1 ¾ cups of granulated sugar until light and fluffy, about 4-5 minutes. Add eggs one at a time, followed by 1 cup of sour cream until smooth.

In a separate bowl, whisk together 3 cups of all-purpose flour, 1 tablespoon of baking powder, and ½ teaspoon of salt. Gradually add to the wet ingredients, folding gently.

Combine ½ cup of granulated sugar with 2 teaspoons of ground cinnamon. Grease and flour Bundt pan. Pour half the batter, then sprinkle half the cinnamon sugar mixture, add remaining batter, and top with the remaining cinnamon sugar.

Gently swirl the batter using a knife about 1-2 inches deep to create a marbled effect.

Preheat oven to 350°F (175°C) and bake for 50-55 minutes. Check with a toothpick; it should come out clean.

Allow the cake to cool in the pan for about 10 minutes, then invert onto a wire rack and cool completely before slicing.