Mousseline Sauce is a luxuriously-rich derivative of Hollandaise Sauce. Whipped cream is folded in at the end to create an extremely “light”, fluffy and elegant sauce.
Before making the sabayon, melt the clarified butter and keep it between 130-140 degrees Fahrenheit (55-60 degrees Celsius).
In a stainless-steel bowl, whisk the egg yolks and cold water together until they triple in volume.
Transfer the bowl over a bain marie that contains barely simmering water. Cook the sabayon, about 1 to 3 minutes, until it doubles in volume and you reach the ribbon stage. Once cooked, remove the sabayon from the heat and whisk for about 20 seconds to prevent the eggs from overcooking.
Discard the simmering water from the bain marie and place a damp cloth over the pot. Place the bowl with the sabayon over top. Make sure the butter is between 130-140 degrees Fahrenheit (55-60 degrees Celsius). While constantly whisking, add the butter to the sabayon in a steady stream. Add enough butter until you achieve a somewhat thick consistency. You will not need all of the butter, but it is better to have more on hand than too little.
Once done, add the lemon juice or gastride (see attached drill-down) to taste and adjust the consistency of the sauce, if needed.
Quickly whip the heavy cream until it forms soft peaks. Fold the whipped cream into the sauce. Season to taste with salt and cayenne pepper and serve immediately.