Bring a small saucepan of salted water to a boil. Add the haricots verts and cook until just tender, about 1 minute. Transfer with a slotted spoon to an ice bath. Drain. In a large skillet, melt the butter over medium-high heat. Add the drained haricots verts and salt and pepper to taste, and cook, stirring, for 3 minutes, or until just heated through and crisp-tender.