In a large bowl, add the masa harina. Start with 1 ½ cups warm water, then slowly add more as you knead.
Mix and knead until the dough reaches a soft, playdough-like texture. It should be smooth and not crack when pressed. If it cracks, add a little more water.
Let the dough rest for 10 minutes to hydrate.
Meanwhile, prepare the filling: add mozzarella cheese to a bowl and mix with a small splash of milk. Mash together until soft and slightly creamy.
Take a portion of dough and roll it into a ball. Flatten it into a disc using your palms.
Place a spoonful of cheese mixture in the center.
Carefully fold the edges over the filling, sealing it completely, then gently reshape into a thick disc. Trim any excess dough if needed.
Heat a skillet over medium heat. Cook each pupusa for about 3 minutes per side. Keep cooking 3 minutes on each side until the desired golden color.
Flip and repeat until both sides are golden brown and slightly crisp, and the cheese is melted inside.