Combine the milk, heavy cream, the first portion of sugar, and salt in a small saucepan/pot. Bring to a boil, stirring occasionally to dissolve the sugar.
When the mixture is boiling, remove from the heat. Set aside momentarily.
In a medium to large bowl, whisk the egg yolks and the second portion of sugar together. While whisking, gradually pour about 1/3 of the hot milk mixture into the bowl with the egg yolk mixture to temper the egg yolks.
Pour the tempered egg mixture into the saucepan with the hot milk mixture.
Cook on low heat, stirring constantly, scraping the bottom and sides of the pot, until the mixture thickens slightly and coats the back of a spoon. (This should be around 175F.) Remove from heat immediately. Be careful not to overcook, else you'll get scrambled eggs!
Strain out any pieces of egg by pouring the mixture through a sieve into a metal bowl. Stir in the vanilla extract.
Place the metal bowl in a larger bowl that has ice in it to quickly cool the vanilla sauce. Stir periodically for 15-20 min.
Transfer vanilla sauce into a container or cover the bowl with plastic wrap and chill overnight.