Bring enough water to fill baking dish to a boil. Place kippers, fully opened and skin side down, in a shallow dish. Fill dish with boiling water. To keep submerged in boiling water, cover fish with parchment paper and a small cutting board. Place something heavy on top to weigh kippers down. Let stand 3 to 4 minutes. Move fish to a serving plate, skin side down, and top with butter, if desired.