Combine blue corn masa harina and salt in a bowl. Add warm water gradually, mixing with your hands until the dough comes together into something that feels like soft clay. It should be pliable and smooth, not sticky and not crumbly. If it cracks at the edges when you press it, add water one tablespoon at a time. Knead gently for 2 minutes.
Cover with a damp cloth and rest for 15 minutes. The rest is not optional. The masa needs time to fully hydrate, and the dough will be significantly more workable after resting.
Pull off a golf-ball sized piece of dough. Press flat between your palms to about 5 to 6 inches wide and a quarter inch thick. Use your hands, not a rolling pin or tortilla press. The hand-pressed shape is part of the authenticity of this preparation. The edges will be slightly uneven. That is correct.
Heat a dry cast iron skillet over medium-high heat until genuinely hot. No oil. Place a flatcake in the dry pan. Cook 3 to 4 minutes per side until dark spots appear and the edges begin to lift. The cake will deepen from grey-blue raw dough to a darker, almost storm-cloud blue-grey as it cooks. Stack finished flatcakes in a clean cloth to keep warm and pliable while you cook the rest.