Ingredients

• 3 large egg yolks

• 1 tablespoon cold water

• 1 teaspoon cornstarch

• 1 teaspoon Dijon mustard

• 1 ½ tablespoons fresh lemon juice, divided

• 1 teaspoon kosher salt, divided

• 12 tablespoons unsalted butter (6 ounces), melted, plus 2 tablespoons cold unsalted butter, divided

• ⅛ teaspoon cayenne pepper

• Warm water, as needed

• 4 cold large eggs

• 4 (1/2-inch-thick) sourdough bread slices, toasted

• 8 thin prosciutto or speck slices (about 4 ounces)

• 2 tablespoons roughly chopped fresh soft herbs (such as tarragon and chives)

• Freshly ground black pepper (optional)