01 - Preheat the oven to 350°F. Grease and flour a Bundt pan thoroughly.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
03 - In a large bowl, cream the butter and granulated sugar together until light and fluffy, about 2–3 minutes.
04 - Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
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; do not overmix.
06 - In a small bowl, mix together the brown sugar and cinnamon for the swirl.
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 remaining batter, swirl mixture, and nuts.
08 - Gently swirl a knife through the batter to create a marbled effect.
09 - Bake for 45–50 minutes, or until a toothpick inserted into the center comes out clean.
10 - Cool the cake in the pan for 15 minutes, then invert onto a wire rack to cool completely. Once cool, dust with powdered sugar before serving if desired.