Soak the leeks in water until they’re clean, and spin them dry in a salad spinner. Heat the butter in an 11-inch pot over medium heat; add the leeks and cook for 10 minutes, stirring occasionally. Add the wine, 1 teaspoon salt and 1 teaspoon pepper and cook for 5 minutes, until the wine almost evaporates and the leeks are tender. Off the heat, mix in the artichokes, toasted bread cubes, chives and tarragon.