French Laundry Béarnaise Mousseline

A classical béarnaise should be as thick as mayonnaise. This recipe is in effect a standard béarnaise with whipped cream added at the end to make it light and pourable: Reduce red or white wine vinegar with chopped shallots, chopped fresh tarragon, and cracked black pepper until the pan is almost dry (the shallots and herbs should still have plenty of moisture). I do this over direct heat in a pan with sloping sides, but you may feel safer cooking it over a water bath. Add egg yolks and half an eggshell of water per yolk. Whip the yolks continuously over the heat; you’ll see them tighten up and eventually form ribbons. When the eggs are thoroughly cooked—the ribbons should be thick, the mixture should taste cooked and be hot—remove the pan from the heat and whisk in clarified butter in a thin steady stream. When you’ve achieved the right consistency (about 8 ounces of butter for every 3 yolks), taste and season with salt and lemon juice, then strain through a chinois. Just before serving, add chopped fresh tarragon, finely minced shallots, and freshly ground black pepper, then stir in whipped cream—about a third as much by volume.