Put the cornflour/cornstarch in a small bowl, add 1 tablespoon water and stir until completely smooth.
Pour the cream into a heavy-based saucepan and heat over a medium heat until warm. Whisk in the cornflour/cornstarch mixture, stirring vigorously for 3–5 minutes until it thickens to the consistency of clotted cream, then remove from the heat and leave to cool