Make the Chenna: Pour the milk into a saucepan and bring it to a full, rolling boil.
Remove from the heat and immediately stir in the lemon juice. Within a minute, the milk will separate into white curds (chenna) and a yellowish liquid (whey). If it doesn't, return it to very low heat for a moment until it does.
Form the Dough: Without draining, add half of the flour directly into the hot pot with the curds and whey. Use a sturdy spoon to mix vigorously until a thick, smooth paste forms.
Let it cool for a few minutes until it's comfortable to handle.
Knead to Perfection: Transfer the warm mixture to a clean work surface.
Add the salt (if using) and gradually knead in the remaining flour.
Knead for about 8 minutes, until the dough is medium-stiff, pliable, and silky smooth. *For a food processor, let the chenna cool for 3-4 minutes, pour it into the processor bowl, pulse to break up the curds, then add the flour and salt with the machine running until a ball forms.
Rest the Dough: Shape the dough into a ball, place it on your work surface, and cover it with an inverted bowl.
Let it rest for at least 30 minutes or up to 3 hours. A longer rest yields a softer chapati. The dough can also be refrigerated for up to 24 hours; just bring it to room temperature before using.
Shape the Chapatis: Briefly knead the rested dough and divide it into 12 equal portions.
Roll each into a smooth ball and keep them covered with a damp cloth.
Working with one ball at a time, flatten it into a 2-inch / 5 cm disc, dip it in dusting flour, and roll it out into a thin, 6-inch / 15 cm circle.
Cook the Chapatis: Preheat a cast-iron skillet or tava over moderately low heat for 3-5 minutes. Slap a rolled chapati between your palms to remove excess flour and place it on the hot surface.
Cook for about 1 minute, until small bubbles appear and the underside has light brown spots.
Puff them Up: Flip the chapati and cook for another 30 seconds. Now, either use tongs to place it directly over a high gas flame until it puffs up like a balloon (a few seconds per side), or press gently on the edges with a folded towel on the skillet to trap steam and encourage it to puff.
Serve and Store: Transfer the hot chapati to a cloth-lined basket to keep it warm and soft.
Brush with melted butter or ghee, if desired.
Repeat with the remaining dough.
Serve immediately for the best experience.