All the frostings!
Many of the frosting recipes are just a list of ingredients with no instructions. I guess frosting technique is just knowledge you were supposed to have if you had any business in a kitchen at all.
I have tried to cross reference (link) cake recipes to their frosting recipes where indicated.
At least one of the ingredients will be hard to find. I don’t think you could find a 10¢ can of pineapple, even on eBay.