pretty sure you get the recipe from shane,,otherwise you can rely on the chance a shadow shawman or whatever they're called [the shadow dudes that appear on 80-119] to drop one
It’s actually a Cooking Recipe but many people get it as a monster drop from Shadow Brutes. Like many other things in the game you could wait about 18 years and maybe it’ll come up from the Traveling Merchant, but who’s got time for that when there’s resting time under a tr—ehh, Fishing that needs doing.
You get the recipe in the mail from Shane at 7 hearts. It needs a periwinkle, void mayo, and flour. You can sometimes buy one from the saloon, traveling cart, or Krobus on Saturday. And they drop from shadow brutes levels 80+