This is the sauce called “crème Anglaise” in French.
Mix the sugar, yolks, and salt in a bowl and whisk (or use a mixer on medium speed) until pale yellow, 3 to 5 minutes.
Prepare an ice bath with a bowl sitting in the iced water.
Heat the milk and vanilla bean together until almost boiling. Slowly whisk the egg yolk mixture into the hot vanilla milk. Transfer to a double boiler and cook over simmering water, stirring constantly, until the custard begins to thicken and coats the spoon.
Remove from heat and 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.
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