Mix together all the ingredients in a saucepan - ensure the cornflour has completely dissolved.
Cook on a medium heat, stirring regularly, for around 5 minutes or until thickened.
Video
Notes
Tip: It is crucial to ensure that the cornflour (cornstarch) has dissolved completely before heating, otherwise it will clump up once you start to cook the mixture. Regular stirring is also necessary to ensure you don't get clumps.