Be very careful with cornflour (cornstarch) as it can easily turn clumpy. Dissolve it first in a tiny splash of water in a small bowl, until you get a small amount of white liquid (make sure it's not too thick). Make sure it has dissolved completely. Add the cornflour mixture carefully a tiny bit at a time, stirring well to ensure it doesn't clump. If the texture of the sauce looks too thick after you've added the cornflour, add extra water.