This dense, mildly sweet, hearty rye bread is ubiquitous in its native Iceland, where it’s traditionally sliced very thin and served with smoked, cured, or pickled foods. That said, it’s also delicious with a simple smear of salted butter. Since it’s leavened with baking powder and baking soda (not yeast), it’s simple to throw together: no kneading, no rising, just mix, bake, and enjoy.