01 - Preheat oven to 350°F. Grease and flour a Bundt pan thoroughly, ensuring all crevices are coated to prevent sticking.
02 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl until evenly distributed. Set aside for later use.
03 - Beat softened butter and granulated sugar in a large bowl until light and fluffy, approximately 2-3 minutes. The mixture should appear pale and increased in volume.
04 - Add eggs one at a time, mixing thoroughly after each addition. Stir in vanilla extract until fully incorporated.
05 - Add flour mixture to the wet ingredients in three portions, alternating with sour cream. Begin and end with flour mixture. Mix just until combined, taking care not to overmix as this can toughen the cake.
06 - Combine brown sugar and cinnamon in a small bowl, mixing until uniform.
07 - Spread half the batter evenly into the 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 create a marbled pattern. Avoid over-swirling which can blend the layers too much.
09 - Bake for 45-50 minutes until a toothpick inserted into the center emerges clean. The top should be golden brown and spring back when lightly touched.
10 - Let the cake cool in the pan for 15 minutes before carefully inverting onto a wire rack. Allow to cool completely to room temperature.
11 - Dust the cooled cake generously with powdered sugar just before serving. Slice and enjoy warm or at room temperature.