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 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 using an electric mixer until light and fluffy, approximately 2-3 minutes.
04 - Beat in the eggs one at a time, mixing thoroughly after each addition. Stir in the vanilla extract until fully incorporated.
05 - Add the flour mixture to the wet ingredients in three alternating additions with the sour cream, beginning and ending with the flour. Mix only until combined to avoid tough texture.
06 - In a small bowl, mix together 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 layers with remaining batter, swirl mixture, and nuts.
08 - Gently swirl a knife through the batter in a circular motion to create a marbled cinnamon pattern throughout the cake.
09 - Bake for 45-50 minutes, or until a wooden toothpick inserted into the center comes out clean and the cake is golden brown.
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.