Meanwhile, combine corn starch and 1/2 cup sugar in medium saucepan. Gradually stir in water until blended. Bring to boil on medium heat, stirring constantly; cook 8 min., stirring constantly. Remove from heat. Add dry jelly powder, butter, lemon zest and juice; stir 2 min. or until jelly is completely dissolved. Refrigerate 30 min. or until slightly thickened, stirring occasionally.