Preheat the oven to 350?F (175?C). Grease and flour a 9x13 inch baking pan.

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

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

In another bowl, whisk together the flour, baking powder, baking soda, and salt.

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

In a separate bowl, combine the brown sugar, flour, melted butter, cinnamon, and walnuts (if using). Mix until crumbly.

Pour half of the cake batter into the prepared pan. Sprinkle half of the topping mixture over the batter.

Spread the remaining batter over the topping, then sprinkle the rest of the topping on top.

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Allow to cool slightly before serving.