01 - Preheat the oven to 350°F. Thoroughly grease and flour a Bundt pan to prevent sticking.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt until well combined. Set aside.
03 - In a large bowl, beat the softened butter and granulated sugar together until light and fluffy, approximately 2–3 minutes.
04 - Add eggs one at a time, mixing thoroughly after each addition. Stir in the vanilla extract until incorporated.
05 - Alternate adding the flour mixture and sour cream to the butter mixture in three additions, beginning and ending with the flour. Mix until just combined, being careful not to overmix.
06 - In a small bowl, combine brown sugar and ground cinnamon, mixing until evenly distributed.
07 - Spread half of the batter into the prepared Bundt pan. Sprinkle with half of the cinnamon mixture and half of the nuts if using. Repeat layers with remaining batter, cinnamon mixture, and nuts.
08 - Gently run a knife through the batter in a swirling motion to create a marbled pattern throughout the cake.
09 - Bake for 45–50 minutes until a toothpick inserted into the center emerges clean.
10 - Allow the cake to rest in the pan for 15 minutes before inverting onto a wire rack. Cool completely before dusting with powdered sugar.