In a bowl, combine the eggs, sugar, milk and vanilla. Combine the flour, cinnamon, baking powder and salt; add to egg mixture and mix well. Stir in the apples and walnuts. Transfer to a greased 13x9-in. baking pan. Bake at 350° for 40-45 minutes or until a toothpick inserted in the center comes out clean. Cool on wire rack. Cut into squares. Serve with whipped topping if desired.