Whisk together the egg yolks, sugar, and salt in a stainless steel mixing bowl until creamy. Set aside.
Put the milk and the vanilla extract or vanilla bean in a heavy-bottomed saucepot. Scald the milk, and slowly whisk it into the egg mixture.
Put the bowl over a pot of simmering water and cook the custard, stirring constantly with a wooden spoon, for about 5 minutes, until it begins to thicken. It should coat the back of the wooden spoon. Do not let it boil.
Cool the custard sauce over an ice bath. Strain and refrigerate until ready to use. Custard sauce will keep for 2 days in the refrigerator.