Preheat oven to 350° Fahrenheit and grease a 9×9″ baking pan.
In a medium mixing bowl, sift together the flour, baking powder, baking soda, and salt. Set aside.
Cream the butter and sugar together until pale and fluffy. Beat in the egg and vanilla. Add the sour cream.
Turn the mixer down to low speed and mix in the flour gradually. Stir just until combined.
In a separate small bowl, combine the sugar, cinnamon, and nuts for the topping.
Spread 1/2 of the batter into the baking pan and spread it by hand so that it covers the bottom of the pan. Sprinkle 1/2 of the brown sugar topping over the top, then spread the remaining batter over the sugar layer. Sprinkle the remaining 1/2 of the sugar mixture over the top.
Bake for 40 minutes, or until the cake springs back when lightly touched. Serve warm or cooled to room temperature.
Will keep in a sealed container for 2-3 days.
This recipe is half of the original recipe. Find the original 9 x 13 inch version here.