The pupusa is a traditional "fast food" sold by street vendors in El Salvador. It's a thick, handmade tortilla stuffed with a savory filling and cooked to a golden brown. Other versions of pupusas, called by different names, are popular throughout Central America. Some favorite fillings include cheese, chicken, and pork skin cracklings.
Combine the masa harina and water in a medium-size bowl. Mix thoroughly until the dough just holds together; it will have the consistency of pie dough.
Divide the dough into 12 balls: six 2-ounce balls and six 3-ounce balls. Set aside.
Place the black pepper, cumin, red pepper sauce, bell pepper, tomatoes, and onion in a food processor or blender; puree until well blended.
Flatten one 3-ounce ball of tortilla dough into a 5-1/2 to 6 1/2-inch circle.
In the center of the tortilla, place 1 tablespoon of the pureed mixture, 1 tablespoon of the mozzarella cheese, and 2 tablespoons of the chicken.
Flatten one 2-ounce ball of the tortilla dough into a 4- to 5-inch circle and place over the larger tortilla, covering the filling. Press the edges of both tortillas together, so they form a sealed pocket. Repeat this process five more times.
Pour 1 teaspoon of oil in medium nonstick skillet and spread it around with a paper towel. Heat the oil; add 1 or 2 pupusas. Cook over medium heat, turning once, until golden brown on both sides, about 8 minutes. Repeat until all the pupusas are cooked.
Substitute 1-1/4 cups of shredded mozzarella cheese for both the chicken and the cheese in the pupusa filling. When filling the tortillas, top the puree mixture with 3 tablespoons of cheese. Continue as directed.