Cook beans in a medium pot of boiling salted water (2 tablespoons salt for 4 quarts water), uncovered, until crisp-tender, 4 to 6 minutes. Drain and plunge into an ice bath to stop cooking. Drain well and pat dry.
Cook bacon in a large heavy skillet over medium heat until crisp. Drain on paper towels, reserving fat in skillet, then crumble. Add butter to fat in skillet along with beans, chestnuts, bacon, and 1/4 teaspoon each of salt and pepper. Cook, stirring frequently, until heated through, 2 to 3 minutes.
Cooks note: Haricots verts can be washed and trimmed 1 day ahead and chilled in a sealable bag lined with paper towels.