There would still be a recipe even if the book was bought by someone else, like her sister. Yuriko was in so much panic and burdened by guilt that she forgot even the very basics.
There's a fine line between aloof eccentricity (like Yugami from Yugami-kun ni wa Tomodachi) and being dumb. This is probably the first chapter were I thought this girl is probably more dumb than just eccentric.