Preheat oven to 350F. Coat a 9-inch square pan with cooking spray.
Cream 5 tablespoons butter and 1 cup sugar. Add egg and beat well. Stir in flour, baking soda, cinnamon, salt, apples and walnuts. Batter will be stiff. Spoon into prepared pan; smooth top. Bake about 35 minutes.
Combine remaining 3 tablespoons butter, 1/2 cup sugar, cornstarch and evaporated milk in a saucepan; bring to a boil over medium heat. Boil 2 minutes, stirring constantly. Remove from heat; stir in lemon juice and vanilla. Pour over hot cake. Serve warm.