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 for later use.
03 - In a large bowl, beat softened butter and granulated sugar with electric mixer on medium-high speed until light and fluffy, approximately 2-3 minutes.
04 - Add eggs one at a time, beating well after each addition until fully incorporated. Mix in vanilla extract.
05 - Add flour mixture to butter mixture in three alternating additions with sour cream, beginning and ending with flour. Mix just until combined; do not overmix.
06 - In a small bowl, combine brown sugar and ground cinnamon, mixing thoroughly until evenly distributed.
07 - Spoon half of batter into prepared Bundt pan. Sprinkle with half of cinnamon swirl mixture and half of nuts if using. Repeat layers with remaining batter, swirl mixture, and nuts.
08 - Gently draw a knife through batter in a circular motion to create marbled swirl pattern, being careful not to scrape the pan sides.
09 - Bake for 45-50 minutes until a toothpick inserted into center comes out clean. The top should be golden brown and spring back when lightly touched.
10 - Allow cake to cool in pan for 15 minutes before inverting onto wire rack. Let cool completely to room temperature.
11 - Once completely cooled, dust generously with powdered sugar. Slice and serve as desired.