Combine corn masa, 1-1/4 cups water, salt and pepper in large bowl until soft dough forms. Gradually add more water, if necessary.
Divide dough into 6 equal balls. Make indentation in center of each ball; fill with about 2-1/2 tablespoons cheese. Push dough up and around cheese to completely enclose. Flatten balls with hands or rolling pin into 5-1/2 to 6-inch discs, making sure dough stays sealed. Cover with damp paper towels until ready to cook.
Spray large skillet or griddle with nonstick cooking spray; heat over medium-high heat until hot. Place pupusas in skillet in batches; cook 2 to 4 minutes or until lightly browned, turning once. Keep warm.