Make Dough: Add masa harina (corn flour) and salt to a large mixing bowl then add the warm water, little by little, while mixing the dough with your hand until you get the consistency of a soft play dough. Stir in butter and chicken bouillon.
Scoop dough into golf-ball-size portions, dipping your hands in an oil and water mixture as needed if the masa is sticking to your hands. Keep dough balls covered with a damp cloth as you work, to keep from drying out.
Form Pupusas: Pat the dough into a pancake in your hand, about 4 inches in diameter. Add a Tablespoon of refried beans in the center and a sprinkle of shredded cheese. Fold the edges up and pinch to close into a ball. Gently pat the dough back and forth between your palms to form it back into a thin pancake.
Cook: Heat a large un-greased skillet or pan over medium heat. Place pupusas on the hot pan and cook for about 2-4 minutes on each side. You will know the pupusas are ready to flip when the edges are set and the bottom is lightly golden.
Bouillon: I prefer bouillon paste, or use chicken granulated bouillon or one cube.Filling: Feel free to try different filling combinations. It is popular to include beans, meat, and cheese.Store leftover masa dough in the refrigerator in an airtight bag or container for 1-2 days. Add a little more warm water to it, as needed, to make it soft before using. Store leftover cooked pupusas in an airtight container in the fridge, and reheat on a hot griddle or skillet.To Freeze uncooked pupusas, "flash freeze" them on a baking sheet for 30 minutes, then store in an air-tight container, separated by parchment, for up to 3 months. Thaw completely in the fridge before cooking and serving.