01 - Preheat oven to 350°F. Grease and flour a Bundt pan thoroughly, ensuring all surfaces 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 an electric mixer until light and fluffy, approximately 2-3 minutes.
04 - Add eggs one at a time, mixing thoroughly after each addition. Stir in vanilla extract until fully incorporated.
05 - Add flour mixture to wet ingredients in three additions, alternating with sour cream. Begin and end with flour mixture. Mix just until combined, being careful not to overmix.
06 - In a small bowl, combine brown sugar and ground cinnamon, mixing until evenly distributed.
07 - Spoon half of the 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 run a knife through the batter in a swirling motion to create a marbled pattern, being careful not to disturb the pan's shape.
09 - Bake for 45-50 minutes until a toothpick inserted into the center comes out clean. The top should be golden brown and firm to the touch.
10 - Allow cake to cool in pan for 15 minutes before carefully inverting onto wire rack. Cool completely, then dust with powdered sugar if desired before serving.