Pupusas need not only have a filling of chicharron. A center full of cheese, mozzarella or other meltable cheese, makes for a delicious treat. While in Honduras I found that cheese-filled pupusas are fairly easy to make if you keep in mind that the cheese can leak out easily if you have not closed the pupusa properly
A little trick to make sure that no cheese escapes: Before rolling/patting the pupusa flat, dig your thumb into the ball of masa as to make a deep impression. Insert cheese or chicharron, then pinch the masa closed around the filling. You now have a ball of masa with filling in the center ready to be flattened and cooked.