Place the barley into a 1 1/2-quart ceramic or glass baking dish with a lid, and add the butter, salt, and boiling water. Stir to combine. Cover the dish tightly with aluminum foil and place the lid on top of the foil. Bake on the middle rack of the oven for 1 hour.
After 1 hour, remove the cover, fluff with a fork, and serve immediately.