4: Turn off the heat, tip in the cornflour and water mixture all at once while whisking. The mixture will thicken almost immediately.
5: Pour into mugs or bowls. Serve hot or warm.
Remarks
If you prefer something a little sweeter, increase the amount of vanilla sugar or add a little caster sugar. If your chocolate stash has run out, you can use cocoa powder instead, but you'll definitely need to add more sugar.