FavSandwich
The Favwich, formatted as FavSandwich (JP: ファボサンド Fabo Sando) in the overworld inventory, is an unused consumable in Chapter 1 of Deltarune. It restores 500 HP, which currently makes it the most powerful single member healing item in the game, not counting items that fully restore the HP of a party member.
Despite what the battle text suggests, the FavSandwich is not guaranteed to heal all HP. During the Knight battle in Chapter 3, if used on a fully SWOONed party member (-999 HP), it cannot heal them to full HP by itself. As the FavSandwich is unobtainable as of Chapter 5, this is not possible to see during normal gameplay.
The FavSandwich has a unique interaction in Chapter 5's Shadow Crystal boss fight. If the ShareFood ACT is used against Pink with this item, it grants 15 Dokis, the largest benefit compared to any other item. This makes the battle instantly transition to the next phase, unless it is the third phase, where there is a maximum of 20 Dokis. It is possible this is for debugging purposes, since the FavSandwich is unobtainable as of Chapter 5.
- The FavSandwich could possibly be a reference to Mother 3's Favorite Food, which fulfills a similar purpose in being an unobtainable consumable item that heals a significantly high amount of HP, while coincidentally being the protagonist's favorite food.