A Royal Recipe
Talk to Gotter at the Riverside Stable. He will ask you to find recipes from a cookbook inside Hyrule Castle so he can learn what the royals had for meals before Calamity Ganon destroyed the place. Note that if you know the recipes, you do not have to go to Hyrule Castle to find them. You can just cook the dishes right at the cooking pot. If you want that fast solution, skip the next few paragraphs. Otherwise, let’s go spelunking in Hyrule Castle!
If you have unlocked the Saas Ko’sah Shrine, fast travel there. It is the easiest, fastest way to get to where the recipe books can be found. Essentially, you want to get to Hyrule Castle’s Docks, which can be found on the west side of Hyrule Castle. Using a raft southwest of Hyrule Castle, go around the moat until you see a break in the rocks and enter.
From the Docks, run up the stairs and down the tunnel until you hit what looks like a dead end.
But it’s not a dead end, it is simply a metal block you can push back using Magnesis. Prepare for a fight, though, because Hyrule Castle is infested with enemies and the Library is no exception.
Once you have cleared out the place, you can start searching. Recipe book #1 is on the lower level of the Library right past the entrance. It contains the recipe for Fruitcake: 2 - 3 of any kind of fruit, Tabantha Wheat, and Cane Sugar.
Recipe book #2 is on the upper level of the library on a stand. It gives the recipe for Monster Cake, which is also used in the sidequest A Parent’s Love. Click the link to learn more if you’d like! The recipe for a Monster Cake is Monster Extract, Cane Sugar, Tabantha Wheat, and Goat Butter.
Cook the dish in any cooking pot and then speak with Gotter. He will ask if he can eat what you made. Go ahead and give him whatever dish you make and he will give you a silver Rupee worth 100 Rupees as thanks. At this point, the sidequest is complete. If you cook the other dish and give it to him again, he will give you another silver Rupee for a total of 200 Rupees.