Sift the flour, baking powder, and salt into a bowl. Slowly add as much yogurt as you need to gather the flour together and make a soft, resilient dough. Knead for about 10 minutes and form a ball [the dough, not you. -S ] Put the ball in a bowl and cover the bowl with a damp dishcloth. Set aside in a warm place for 1 1/2 - 2 hours. Knead the dough again and divide into nine equal parts. Keep them covered. Heat a cast-ironed skillet or griddle over a lowish flame. Preheat the broiler. Take one of the parts of dough and make a ball out of it. Flatten it and then roll it out on a lightly floured surface until you have a round that is about 1/8 inch thick. When the skillet is very hot, pick up the naan and slap it onto the heated surface. Let it cook slowly for about 4 to 5 minutes. It will either puff up completely or partially. Now put the whole skillet under the broiler for 1 to 1 1/2 minutes or until the puffing-up process completes itself and there are a few reddish spots on the naan. Remove the naan with a spatula and brush with butter if you like. Make all of the naans this way, keeping them stacked and covered with a clean dishcloth. Serve hot. If you wish to have the naans later, wrap them in a plastic bag when they have cooled. Before you eat, wrap as many as you need in aluminum foil and heat in a 400 degree oven for 15 minutes.