Heat the grill. Set the grill rack 4 inches (10 cm) from the heat. Place the butter in a rectangular dish large enough to hold the salmon. Let it melt under the grill. Remove the dish from the heat, place the salmon in the butter, turning to coat. Grill until firm and lightly golden on top, about 4 minutes. Turn them over, baste with the butter and grill for 4 minutes. Remove and test the salmon -- if it comes away from the bone easily, it is done.