The single most nerve-racking thing about making this simple, elegant custard sauce is that it can overcook and curdle. Grace Parisi prevents this by preparing a cold-water bath and setting it near the stove before she even cracks an egg.
How to Make the Ultimate Umami-Bomb Vegetarian Demi-Glace
2 cups half-and-half or whole milk
1 vanilla bean, split lengthwise
1/2 cup sugar
4 large egg yolks, at room temperature
Set a large fine strainer over a medium bowl and set the bowl in a shallow pan of cold water.
In a large saucepan, combine the half-and-half and vanilla bean and cook over moderately low heat just until small bubbles appear around the rim, about 5 minutes.
In another medium bowl, whisk the sugar and egg yolks just until combined. Whisk in half of the hot half-and-half in a thin stream. Pour the mixture into the saucepan and cook over moderate heat, stirring constantly with a wooden spoon, until the sauce has thickened slightly, 4 to 5 minutes. Immediately strain the sauce into the bowl in the cold water bath to stop the cooking. Scrape the vanilla seeds into the sauce. Serve right away or refrigerate until chilled.