01 - Preheat the 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 the flour, baking powder, baking soda, and salt until well blended. Set aside for later use.
03 - In a large bowl, beat the softened butter and granulated sugar together with an electric mixer until light and fluffy, approximately 2-3 minutes.
04 - Add the eggs one at a time, mixing thoroughly after each addition. Stir in the vanilla extract until fully incorporated.
05 - Gradually add the flour mixture to the wet ingredients in three portions, alternating with the sour cream. Begin and end with the flour mixture, mixing only until combined. Avoid overmixing to maintain a tender texture.
06 - In a small bowl, mix the brown sugar and ground cinnamon until evenly combined.
07 - Spoon half of the batter into the prepared Bundt pan. Sprinkle with half of the cinnamon swirl mixture and half of the nuts if using. Repeat with the 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 coating.
09 - Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean. The cake should be golden brown and spring back lightly when touched.
10 - Allow the cake to cool in the pan for 15 minutes, then carefully invert onto a wire rack to cool completely. Dust with powdered sugar before serving if desired.