Preheat the Oven: Start by preheating your oven to 350°F (175°C). Prepare a 9-inch round cake pan by greasing and flouring it.
Mix Dry Ingredients: In a mixing bowl, combine the flour, baking powder, baking soda, and salt — set this mixture aside.
Cream Sugar and Butter: In a separate bowl, beat the sugar and softened butter together until the mixture is light and fluffy.
Add Eggs and Vanilla: Incorporate the eggs one by one into the butter mixture, mixing well after each addition. Stir in the vanilla extract and sour cream until smooth.
Combine Ingredients: Gradually add the flour mixture to the wet ingredients, mixing until just combined.
Prepare Topping: In a small bowl, mix together the brown sugar, chopped nuts (if using), and cinnamon.
Layer the Batter: Pour half of the batter into the prepared pan, sprinkle half of the topping mixture evenly over it. Pour in the remaining batter and then add the rest of the topping.
Bake: Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Allow it to cool for a few minutes before serving.