Melt butter or margarine in 9x13" pan - mix in cake mix and one beaten egg. Mix in the pan. Spread in a thin layer. Next: Beat together 1 - 8 ounce package of cream cheese, 2 eggs, 1 tsp. vanilla, 1# of powdered sugar. Pour over the spread mixture. Sprinkle with chopped walnuts or pecans if desired. Bake for 40 minutes at 350 degrees. Cut in squares while warm (not hot).