In a small pot, slowly and carefully bring the heavy cream, milk, sugar and the salt to a simmer, until the sugar is completely dissolved. This will take you between 5 and 10 minutes. When the sugar is dissolved, take the pot from the heat.
In a bowl, whisk the egg yolks. While you constantly whisk, whisk a third of the warm cream-milk-sugar mixture into the egg yolks. This will bring the yolks to the right temperature while ensuring the yolks are mixed thoroughly.
Return the yolk-cream mixture to the pan, and place on a medium-low heat. While you stir the mixture, heat it to 76oC/170oF. At that point, it is thick enough to cover the back of a spoon. Remove from the heat.
Strain the custard through the sieve into a bowl and allow to cool to room temperature. Cover with cling film and allow to cool for 4 hours, or overnight.