Measure out heavy cream, milk, and sugar in a pot.
Cut the vanilla bean in half and with a dull part of the knife scrape all the seeds out and add to the above mixture.
In a separate bowl add egg yolks and cornstarch and whisk the mixture until nice smooth and no clumps are visible.
Heat up the milk and cream mixture on medium heat with constant whisking until it starts to boil. Don’t let it boil vigorously. Take it off the heat.
With continuous mixing add a ladle or two of hot milk/cream mixture to the egg yolk mixture and whisk to slowly heat up the eggs so that they do not overcook immediately.
Slowly and with continuous whisking add the egg mixture to the milk/cream mixture. Put back on the stove on low heat and stir for a minute until the mixture and creamy and sticks to the spatula. If you run the finger over the spatula the custard shouldn’t drip over the scraped line. You don’t want the mixture to boil, but just to have enough heat to cook the eggs.
Transfer the custard to the clean bowl, cover with plastic wrap and put the mixture on the ice bath.
Once it is cooled enough not to burn the finger, place the bowl in the fridge, preferably overnight to cool completely.
Once the custard is completely cooled, take the plastic wrap off and churn it according to appliance instructions.