1. You will need a two-litre (seven-cup) cast-iron pot with a tight-fitting lid. Using the butter or margarine, generously grease the inside of both the pot and the lid.
2. Sieve the flour and salt into a bowl, and mix in the sugar and yeast. Gradually add enough warm water to form a dough. The amount will vary according to the humidity in the air. Knead until firm but elastic. Place the dough in a lightly oiled bowl, cover with a damp cloth, and set aside in a warm spot until doubled in size (about one hour).
3. Knock down the dough and knead again; then roll it into a ball and place in the prepared pot. Cover with the lid and set aside until the dough has once more doubled in size. Once the dough has risen almost to the top of the pot, put the pot on the stove over low heat and cook, covered, until the bread has set (about 30 minutes).
4. Take the bread out of the pot, turn it over, return to the pot, cover, and cook for another 30 minutes so that the bread has a golden-brown crust on the top and bottom.
5. Traditionally this bread is made over a fire. If you do this, place some coals on top of the pot lid so the bread cooks evenly on all sides.