Preparation In a processor, purée 1/2 cup mayonnaise, 1/3 cup buttermilk, 1/4 cup chopped fresh chives, 1/4 cup coarsely chopped flat-leaf parsley, 1 tablespoon chopped fresh tarragon, 1 tablespoon fresh lemon juice, 2 anchovy fillets packed in oil (drained, chopped), and 1 chopped garlic clove until smooth. Season with kosher salt and freshly ground black pepper. DO AHEAD: Can be made 2 hours ahead. Cover and chill.