Virginia Willis' Deviled Eggs

INGREDIENTS

• 1/3 cup mayonnaise
• 2 tablespoons finely chopped fresh tarragon, chives, or chervil, plus leaves for garnish
• coarse salt and freshly ground white pepper
• 1 tablespoon dijon mustard
• 1 pinch cayenne pepper
• 12 large eggs (about a week old if possible -- they'll be easier to peel)
• 2 tablespoons unsalted butter, at room temperature