Split the whole vanilla bean in half and scrape the vanilla seeds with the tip of the small knife. In a medium bowl, whisk egg yolks and icing sugar with a hand whisk.

Pour milk into a heavy medium saucepan and add vanilla seed with an empty pod. Bring the milk over medium heat to a boil. Remove milk from the heat, cover the pot with a lid, and infuse the milk for 10-20 minutes. Remove the empty pod.

Temper the egg yolks by gradually pouring 1/3 of the hot milk mixture through a fine sieve to the egg yolk mixture, stirring constantly. Return the eggs and milk mixture back to the saucepan (with the rest of the milk) over low heat and stir constantly with a hand whisk or a wooden spoon.

The custard is cooked when you draw a horizontal line with your finger in the spoon that it does not close (this is called cooking à la nappe) or when the instant-read thermometer reaches a temperature of 180°F (82°C). Don't exceed a temperature of 185°F (85°C); otherwise, the eggs will coagulate, making visible lumps in the custard. 

Pro tip: One of the best ways to avoid overcooking the custard is to remove it from the heat once it reaches 174°F (79°C). While in the pot, it will continue cooking and raise the temperature by 5 or 6°C. Using a cooking thermometer, in this case, is a must.

Optionally, strain the vanilla custard through a fine mesh sieve to remove any possible lumps. Transfer custard from the hot pot to a dish to stop cooking. Use it immediately or cover it with a piece of plastic wrap in contact (it has to touch the surface of the sauce), and refrigerate.