01 - Preheat the oven to 350°F. Grease and flour a Bundt pan thoroughly to prevent sticking.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside for later use.
03 - In a large bowl, cream the butter and granulated sugar together until light and fluffy, approximately 2-3 minutes.
04 - Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract until fully incorporated.
05 - Add the flour mixture to the wet ingredients in three additions, alternating with the sour cream, beginning and ending with the flour. Mix just until combined to avoid overmixing.
06 - In a small bowl, mix together the brown sugar and 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 figure-eight motion to create a marbled cinnamon pattern throughout the cake.
09 - Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean. The top should be golden brown.
10 - Allow the cake to cool in the pan for 15 minutes, then carefully invert onto a wire rack to cool completely.
11 - Once completely cooled, dust generously with powdered sugar before slicing and serving.