Add milk, sugar, vanilla, and salt to a pot and warm on the stove until steaming over low to medium heat.
Meanwhile, add egg yolks to a medium sized bowl and mix until frothy.
Temper the eggs by VERY slowly adding the warm milk mixture to the eggs. Beat the eggs continuously while you do this (very important!). The slower the addition of the milk the better- you don't want them to get too hot or they will scramble.
Put egg and milk mixture back in pot on the stove, and heat over low to medium heat. Stir frequently. This mixture will cook until it forms a thick, custardy consistency- it should stick to the back of a spoon and trickle off gradually.
While the milk mixture is heating, add the heavy whipping cream to a stand mixer or bowl (using an electric hand mixer). Whip until thickened, but before it turns to a full whipped cream consistency.
Once the custard mixture is done, it's time to add it to the heavy whipping cream. If you notice cooked egg pieces in the custard (either from tempering incorrectly, or not stirring enough when on the stove), strain through a mesh sieve. Add to whipped cream mixture and stir together.
Pour mixture into a Tupperware container, and lay a piece of saran wrap over, touching the custard mixture. This prevents a film from forming. Store in the fridge overnight.