01 - Preheat the oven to 350°F. Grease and flour a Bundt pan thoroughly, ensuring all crevices are coated to prevent sticking.
02 - In a medium bowl, whisk together the flour, baking powder, baking soda, and salt until evenly distributed. Set aside for later use.
03 - In a large bowl, beat the softened butter and granulated sugar together until the mixture becomes light and fluffy, approximately 2–3 minutes.
04 - Add the eggs one at a time, mixing thoroughly after each addition. Stir in the vanilla extract until fully incorporated.
05 - Gradually add the flour mixture to the wet ingredients in three additions, alternating with the sour cream. Begin and end with the flour mixture, mixing just until combined. Avoid overmixing to prevent tough texture.
06 - In a small bowl, combine the brown sugar and ground cinnamon, mixing until uniform in color and texture.
07 - Spoon half of the batter into the prepared Bundt pan, spreading evenly. Sprinkle with half of the cinnamon swirl mixture and half of the chopped nuts if using. Repeat with the remaining batter, swirl mixture, and nuts.
08 - Gently run a knife through the batter in a swirling motion to create the marbled cinnamon pattern. Take care not to disturb the layered structure excessively.
09 - Bake for 45–50 minutes, or until a wooden toothpick inserted into the center emerges clean or with only a few moist crumbs attached.
10 - Allow the cake to cool in the pan for 15 minutes before carefully inverting onto a wire rack. Let cool completely to room temperature.
11 - Once completely cooled, dust the cake generously with powdered sugar using a fine-mesh sieve. Slice and serve with coffee or tea.