Mix the masa harina and salt in a large bowl. Gradually add the warm water and knead with your hands until a smooth, moist dough forms that resembles playdough—it should be soft enough to shape without cracking but not so wet that it sticks to your fingers.

Divide the dough into 8 equal portions and roll them into smooth balls. Combine the shredded cheese and minced jalapeños in a small bowl, tossing until evenly distributed.

Wet your hands slightly to prevent sticking. Flatten one ball of dough into a 4-inch-wide round. Place the cheese mixture in the center of the dough, then fold the edges up and over the filling to enclose it completely, pinching the top to seal.

Gently press the filled ball between your palms to flatten it back into a 4-inch disk, about 1/2-inch thick. Be careful not to press too hard, or the cheese might burst through the sides.

Heat a cast-iron skillet or griddle over medium heat until a drop of water flicked onto the surface sizzles instantly. Lightly brush the surface with vegetable oil.

Place the pupusas in the skillet and cook until the bottoms are deeply browned and charred in spots, 3 to 5 minutes. Flip and cook until the second side is browned and the pupusas have puffed slightly.

Serve immediately while the cheese is molten and the exterior is crisp.