In a mixing bowl, cream sugar and shortening. Add egg and mix well. Add the dry ingredients; mix well. Fold in the apples and walnuts. Spread in a greased 20 cm square baking pan. Bake at 180ºC or 35 to 40 minutes or until cake tests done. For sauce, combine sugar, cornstarch and cream in a saucepan. Bring to a boil over medium heat; boil for 2 minutes. Remove from the heat. Add butter and vanilla; stir until butter is melted. Serve warm with cake.