Warm yogurt and stir in milk, until thoroughly mixed. Remove from heat.
Add baking soda, sugar, 2 tbs butter, eggs and dry yeast.
Sift together the flour and salt. Make a well in the flour and gradually add the yogurt mixture.
Knead the flour 15 to 20 mins until smooth and elastic. Brush dough with some of the remaining butter. Cover with a warm, damp cloth and set aside in a warm place for 3 hours until the dough had risen to twice its size.
Dust hands with flour. Knead the dough again for a few mins and divide into 8 balls. Roll each ball into a 10-in pancake. Pull each pancake gently to give it an oval shape. Cover with damp cloth 20 mins.
Heat a griddle until very hot. Mix the remaining 2 tbs butter with poppy seed. Brush one side of each nan with the mixture and the other side with warm water.
Place the warm-water side on the griddle for 1/2 minute. Remove from the griddle and place nan under broiler for about 2 mins. Serve with curries and tandoori dishes.