Take a heatproof mug or ramekin, and place 2 tbsp caster sugar and 1 tbsp water in the bottom. Microwave on high for 60 seconds, then in 10-second bursts until the sugar caramelises.
Remove and set aside to cool slightly. Don’t worry if it hardens.
In a separate mug of the same size fill it halfway with milk, crack in 1 egg, add 2 tbsp of caster sugar and a little vanilla essence/paste. Stir well.
Pour this into your caramel mug and place back into the microwave for 60 seconds, then cook again in 30-second increments until the custard is set but not boiling.
Chill for half an hour in the freezer or at least 1 hour in the fridge.
