Trim, but don't peel, carrots. Cook carrots in boiling water for 5 minutes; drain. Rinse under cold water. Remove skins by rubbing carrots gently with a clean, dry paper towel. Bring vinegar and brown sugar to a boil in a medium skillet; cook 6 minutes or until reduced to 1/4 cup. Add carrots; increase heat to high, and cook 2 minutes or until sauce is syrupy and coats carrots. Stir in butter and kosher salt.