Montpelier Butter Sauce

Recipe courtesy Jeramiah Tower via the New York Times
 
Serves 6-8

• ​6 leaves spinach
• Leaves from 1/2 bunch watercress (1 1/2 cups of leaves)
• 2 tablespoons fresh flat parsley leaves
• 2 tablespoons fresh chervil leaves
• 2 tablespoons chopped fresh chives
• 1 tablespoon fresh tarragon leaves
• 2 shallots, chopped
• 2 cornichons, rinsed and chopped
• 4 anchovy fillets
• 2 tablespoons capers
• 1 garlic clove, peeled
• ¼ teaspoon cayenne pepper
• 3 hard-cooked egg yolks
• 2 large raw egg yolks
• ¼ pound unsalted butter, at room temperature, cut into 1-tablespoon pieces
• ½ cup extra-virgin olive oil
• 1 teaspoon white-wine vinegar
• Salt and freshly ground white pepper

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.