In a mixing bowl, combine masa harina and salt.
Gradually add warm water, mixing until a dough forms.
Knead the dough for about 5 minutes until smooth.
Divide the dough into 8 equal balls.
Flatten each ball into a disc about ½ inch thick.
Place a tablespoon of refried beans and cheese in the center.
Fold the edges over the filling to seal.
Gently flatten the stuffed disc back into a patty shape.
Heat a skillet over medium heat and cook each pupusa for about 3-4 minutes per side.
Serve hot with curtido and salsa.