Combine water,butter and salt and bring to a boil. Remove from heat,add flour all at once. Stir vigorously until the mixture leaves the sides of pan and forms a ball around the spoon.
Mix sugar, corn starch and yolks in saucepan. Scald(or boil) your milk and pour it over yolk mixture, stir rapidly with whisk. Cook over low heat DON'T LET THIS PART BOIL. when it has become thick remove from heat. Cool slightly before putting inside puffs.