Beat the sugar into the butter until fluffy. Beat in the egg. Beat in the sour cream and vanilla. Fold in the flour until well blended. Spread the batter evenly into an 8 1/2 inch round cake dish. Place the cake on a microwave-proof cereal bowl in the oven. Cook on medium for 6 minutes.

Meanwhile, make the topping by placing the butter in a medium mixing or food processor bowl. With a pastry blender, 2 knives, or processor, cut in the flour, brown sugar and cinnamon. Stir in the nuts.