This is the sauce called crème anglaise in French (and the basis for that most sublime drink, egg nog).
Mix the sugar, yolks, and salt in a bowl and whisk (or use a mixer on medium speed) until pale yellow, about 5 minutes.
Prepare an ice bath with a metal bowl sitting in the iced water.
Heat the milk and vanilla bean together until almost boiling, and pour slowly into the yolk mixture while still whisking. Put the milk and egg mixture in a double boiler and cook over simmering water, stirring constantly, until the custard begins to thicken and coats the spoon.
Remove the custard from the heat and immediately pour into the bowl sitting in the ice. Stir constantly with a spoon to prevent the custard from overcooking (curdling) and forming a skin when it cools. Strain and serve, or cover and refrigerate for up to 3 days.
To make flavored custards, infuse nuts, fresh ginger, cinnamon sticks, star anise, fennel seeds, or orange or lemon rind into the milk and follow the infusion directions for coconut custard (below).
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