Heat a cast iron griddle for at least 2 to 3 minutes (test with water to ensure heat). Place one dough oval in the griddle. After a couple of minutes, it should puff a little at spots. Quickly turn on the other side and sprinkle with a few nigella seeds. Press the seeds gently to set into the dough. Cook for a couple of more minutes—the bread should be puffy and well browned and lightly charred at spots. Repeat with remaining dough disks. If desired, drizzle with a little butter before serving.