In a large bowl, combine the flour, sugar, baking soda, cinnamon and salt. Combine egg and apples; add to flour mixture and mix well. Stir in walnuts.
Transfer to an 8-in. square baking dish coated with cooking spray. Bake at 350° for 25-30 minutes or until a toothpick inserted near the center comes out clean.
For sauce, in a small nonstick saucepan, combine the brown sugar, flour and salt. Gradually add water; stir until smooth. Cook and stir over medium heat until mixture comes to a boil. Cook and stir 1-2 minutes longer or until thickened. Remove from the heat; stir in butter and vanilla. Serve with warm apple dessert. Yield: 9 servings.