Recipe courtesy Jeramiah Tower via the New York Times
Blanch the spinach, watercress, herbs and shallots in boiling water for 1 minute. Drain, refresh under cold water and squeeze dry. Place the mixture in a food processor. Add the cornichons, anchovies, capers, garlic, cayenne and salt and pepper. Process to a smooth paste. Add the egg yolks, cooked and raw, and the butter and process again until thoroughly mixed.
If the butter is still a little chunky (the sauce should be glossy and smooth as velvet), transfer the mixture to a blender and beat in the oil in a thin steady stream while the blender is running. If the mixture is perfectly smooth in the food processor, transfer the butter to a bowl and whisk in the oil by hand. Beat in the vinegar and adjust salt and pepper to taste.