Fill a pot with about 3 inches water and bring it to a simmering boil.
In another heatproof bowl, whisk together the milk, cream, eggs, sugar, cornstarch and vanilla extract.
Place the bowl with the whisked ingredients over the pot.
Continue whisking the custard till it thickens, this might take between 20 mins to 30 mins. Whisk constantly.
The desired texture should be when you are able to scoop the custard up with the whisk and it drops back slowly and heavily.
This was just off the heat, it will continue to thicken as it cools.
Let it cool, whisk occasionally and cover the top with cling film before storing in the fridge to cool completely. The cling film prevents a hard crust forming on top.
Note: You can substitute thick cream for ordinary cream, you'll still get the same result except the whisking process might take longer.