Preheat your oven to 350°F (175°C). Grease and flour a 9-inch square baking pan.

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

Beat in the eggs one at a time, then stir in the vanilla extract.

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

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

Spread half of the batter into the prepared pan.

In a small bowl, combine the brown sugar and cinnamon. Sprinkle half of this mixture over the batter in the pan.

Spread the remaining batter over the top, and then sprinkle with the remaining brown sugar mixture and the chopped walnuts.

Bake in the preheated oven for 35 to 40 minutes, or until a toothpick inserted into the center of the cake comes out clean.

Allow the cake to cool in the pan on a wire rack before cutting into squares and serving.