In a large bowl, combine the first six ingredients. In a small bowl, combine the egg, buttermilk, applesauce, molasses and butter; mix well. Stir into dry ingredients just until moistened.
Coat a 10-in. fluted tube pan with cooking spray and dust with flour; add batter. Bake at 350° for 35-40 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack to cool completely.
In a small saucepan, combine sugar and cornstarch. Stir in water until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat. Stir in the butter until melted. Add the lemon juice, lemon zest and food coloring if desired; mix well. Serve with cake.