Heat all but half a tin of the evaporated milk with the chocolate in a microwave until melted — 4 minutes on full power for me.

Blend the cornflour with the remaining evaporated milk until smooth (i.e. not physics-defying cornflour paste) and add to the mix. Return to the microwave, whisking every two minutes until the mix is thick. It needs to be a bit thicker than custard; this usually takes 8-10 minutes of cooking and stirring for me.

Add caster sugar to taste, allow to cool, then cover and leave to set in the fridge.