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, beating thoroughly after each addition. Incorporate vanilla extract and mix until fully combined.
05 - Add flour mixture in three alternating additions with sour cream, beginning and ending with flour. Mix gently until just incorporated, taking care not to overmix the batter.
06 - Combine brown sugar and ground cinnamon in a small bowl, mixing until uniform in texture and color.
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. Take care not to disturb the layered structure excessively.
09 - Bake for 45–50 minutes until a toothpick inserted into the center emerges clean or with only a few moist crumbs. The top should be golden brown and spring back when touched.
10 - Allow cake to cool in the pan for 15 minutes before carefully inverting onto a wire rack. Cool completely to room temperature before finishing.
11 - Dust cooled cake generously with powdered sugar using a fine-mesh sieve. Slice and serve with coffee or tea.