Preheat oven to 350°F (175°C). Grease a 9×9-inch baking pan or Bundt pan and set aside.

In a large mixing bowl, cream together butter and sugar until light and fluffy.

Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.

In a separate bowl, whisk together flour, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, alternating with sour cream, starting and ending with flour. Mix until just combined.

In a small bowl, mix together brown sugar, cinnamon, and nuts for the cinnamon swirl.

Pour half of the cake batter into the prepared pan, then sprinkle half of the cinnamon-sugar mixture on top.

Add the remaining batter and smooth it out, then sprinkle the remaining cinnamon-sugar mixture over the top.

Bake for 40–50 minutes (if using a square pan) or 50–60 minutes (for a Bundt pan), until a toothpick inserted in the center comes out clean.

Let cool for 10 minutes in the pan before transferring to a wire rack. Slice and serve warm or at room temperature.