Custard, Boiled

A stirred custard sauce under an old name — nothing here is boiled, and boiling is exactly what ruins it. The method is sound; the details below are what the original assumes you already know.

• "Boiled" means nothing of the kindThe custard must never reach a simmer or the yolks scramble into grains. Held in a double boiler over water that is barely trembling, it thickens between 170 and 180F — the point at which it coats the back of a spoon and a finger drawn through leaves a clean track.
• Return it to the heatThe instruction to pour the hot milk over the yolks is tempering. After that the mixture goes back into the top of the double boiler and is stirred constantly, reaching into the corners, until it thickens. It will take roughly 8 to 12 minutes.
• The salt is highHalf a teaspoon in a pint of sweet custard reads as salty to a modern palate. A generous pinch, or up to 1/4 teaspoon, is closer to current taste.
• Sugar and flavouringTwo tablespoons makes a barely sweet sauce, appropriate for pouring over an already-sweet pudding. If it is to be eaten alone, three or four is more usual. This recipe leaves flavouring out entirely; a split vanilla bean scalded in the milk, or a teaspoon of extract stirred in off the heat, is the standard addition.
• Remove quicklyThat phrase carries real weight. Pour the custard immediately through a fine sieve into a cold bowl — the sieve catches any threads of set egg, and the transfer stops the residual heat of the pan from carrying it past the point of no return.
• If it curdlesA custard that has just begun to grain can sometimes be saved by whisking it hard off the heat, or blitzing it briefly with a stick blender, then straining. One that has gone frankly lumpy is finished.
• Yield and keepingAbout two cups, enough to sauce six modest servings of pudding. Cover the surface with film to stop a skin forming, chill, and use within two days.
• The three whitesThe recipe leaves them over. They keep several days covered in the refrigerator and freeze well.