In a kettle of boiling salted water boil the haricots verts for 4 to 6 minutes, or until they are crisp-tender, drain them, and plunge them into a bowl of ice and cold water to stop the cooking. Drain the beans again and pat them dry. The beans may be prepared up to this point 1 day in advance and kept wrapped in dampened paper towels in a plastic bag and chilled. Just before serving, in a large skillet melt the butter over moderately high heat and in it toss the haricots verts until they are heated through. Season the haricots verts with salt and pepper.