A wineskin of 825 ml of permanent ink Milk gives Carl, enough for several chonky books and impossible to fade.
After lamenting that her own best recipes were lost because she wrote them in an ink that faded, Milk pulls a wineskin from her inventory and hands it to Carl. The item description lists 825 ml — enough ink for several chonky books — with a warning that the stuff stains, that nothing will ever make it fade, not even the distance of a time-death star.
Paired with the Adept's Fountain Pen and Milk's pointed reminder that anything Carl writes in Book of Voodoo will be given to everyone who already has a copy, the gift is plainly a suggestion rather than a souvenir. Donut mentions Carl's other recipe book, the goblin-chicken-bearing Dungeon Anarchist's Cookbook, and Milk's answer makes the point explicit: sometimes it takes failure or two to learn how to properly prepare a meal, because no single recipe is a feast in itself.
A wineskin holding 825 ml of deeply staining, permanent ink.
The permanent ink Carl will need to write something that lasts.