In large bowl, mix pupusa masa ingredients by hand. Stir until soft, smooth dough forms. Using wet hands, shape mixture into 1/3-cup balls, shaping each into 4-inch circle, 3/4 inch thick. Place 1 tablespoon filling mixture in center of each circle and fold in half, sealing edges. With wet hands, carefully flatten and shape back into 4-inch circle, 3/4 inch thick. Wet hands, as needed, to prevent dough from cracking and sticking to hands. Cover pupusas with waxed paper while making remainder of pupusas.