01 - Preheat oven to 350°F. Grease and flour a Bundt pan thoroughly, ensuring all crevices are coated to prevent sticking.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt until well combined. Set aside.
03 - In a large bowl, beat softened butter and granulated sugar together until light and fluffy, approximately 2-3 minutes.
04 - Beat in eggs one at a time, mixing thoroughly after each addition. Stir in vanilla extract until incorporated.
05 - Add flour mixture to wet ingredients in three additions, alternating with sour cream. Begin and end with flour mixture, mixing just until combined. Avoid overmixing to prevent tough texture.
06 - In a small bowl, combine brown sugar and ground cinnamon until evenly mixed.
07 - Spoon half the batter into prepared Bundt pan. Sprinkle with half the cinnamon swirl mixture and half the nuts if using. Repeat layers with remaining batter, swirl mixture, and nuts.
08 - Gently drag a knife through the batter in a circular motion to create a marbled cinnamon swirl pattern.
09 - Bake for 45-50 minutes until a toothpick inserted into the center comes out clean or with just a few moist crumbs.
10 - Let cake cool in pan for 15 minutes. Invert onto wire rack to cool completely. Dust with powdered sugar before serving if desired.