01 - Preheat oven to 350°F. Thoroughly grease and flour a Bundt pan to prevent sticking.
02 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl. Set aside for later use.
03 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 2-3 minutes.
04 - Incorporate eggs one at a time, mixing thoroughly after each addition. Stir in vanilla extract until fully combined.
05 - Add flour mixture in three additions, alternating with sour cream. Begin and end with flour mixture. Mix only until just combined to avoid developing gluten.
06 - Mix brown sugar and ground cinnamon in a small bowl until evenly distributed.
07 - Spoon half the batter into prepared Bundt pan. Sprinkle with half the cinnamon mixture and half the nuts if using. Repeat layers with remaining batter, cinnamon mixture, and nuts.
08 - Gently draw a knife through the batter in a swirling motion to marble the cinnamon layers.
09 - Bake for 45-50 minutes until a toothpick inserted into the center comes out clean.
10 - Allow cake to cool in pan for 15 minutes. Carefully invert onto wire rack to cool completely.
11 - Once completely cooled, dust with powdered sugar if desired before slicing and serving.