Combine first 5 ingredients in a small bowl; stir well. Press into a 9 inch pie plate coated with vegetable cooking spray, reserving 1 tablespoon crumb mixture. Bake at 350ºF. for 10 minutes or until browned. Let cool on a wire rack. Sprinkle gelatin over 1/4 cup water in a small bowl; set aside. Combine 1/3 cup sugar and cornstarch in top of a double boiler; stir well. Stir in lemon juice and next 4 ingredients; cook over simmering water 8 minutes or until thickened, stirring constantly. Pour into a large bowl; stir in gelatin mixture, yogurt, vanilla, and salt. Let cool. Beat egg whites (at room temperature) at high speed of an electric mixer until soft peaks form; gradually add remaining 1/3 cup sugar, beating until stiff peaks form. Fold 1/4 of egg white mixture into gelatin mixture; fold remaining egg white mixture into gelatin mixture. Spoon mixture into prepared crust and sprinkle reserved 1 tablespoon crumb mixture around edge. Chill 4 hours or until firm. Garnish with a lemon slice if desired. Yield: 8 servings