Combine sugar, cornstarch and salt in saucepan; add water. Cook over medium heat, stirring constantly, until thickened and bubbly; Continue cooking 1 minute longer. Blend half the hot mixture into a deal. Blend into mixture remaining in saucepan. Cook 1 minute stirring rapidly. Remove from heat; add butter or margarine and lemon rind and juice; cool. Spread between cooled cake layers.