Sieve 500 g millet flour and 0.3 g salt into a wide mixing bowl. Make a hollow in the middle and gradually pour in enough water to bring the ingredients together into a firm dough.
Transfer the mixture to a work surface dusted with extra flour and knead thoroughly for 5 minutes. Place the dough into a bowl greased with a small smear of the 77.5 g melted butter, cover with a damp cloth, and let it rest in a warm area for 30 minutes.
Split the rested dough into 8 uniform portions, roll them into smooth spheres, and let them rest beneath the damp cloth for 5 minutes.
Flatten each sphere between your palms into a thin, round disc.
Warm a heavy skillet or tawa over medium heat until thoroughly hot.
Lay a dough disc onto the dry, hot surface and cook for 1 minute. Turn it over, pressing down gently with a clean dry cloth, and cook until it puffs up, browns with dark spots, and crisps at the rim.
Transfer the cooked flatbread to a plate and keep it warm while repeating the process for the remaining portions, then brush each with the remaining melted butter to finish.