place potatoes in large saucepan, cover with water and add salt. bring to boil and cook until tender about 15 minutes. drain and mash. add egg yolk and cornstarch. mix well and set aside. saute onions in margarine until golden. add cheeses and chile. mix well. shape potato mixture into 10 balls putting some of the chile-cheese mixture in the center of each ball. refrigerate for 30 minutes. flatten balls slightly and saute in hot oil until well browned on both sides.