Fill a medium (3-quart) saucepan three-quarters full of cold water. Set over high heat, and bring to a boil. Add salt and beans. Cook until water returns to a boil and beans are tender, 4 to 5 minutes. Remove from heat, and drain.
Immediately return beans to saucepan. Add butter and zest. Toss; season with salt and pepper. Transfer to a serving bowl; serve warm.