Actually, my reading is the childish single-sound affirmation derived from「うん.」and having one's mouth full. I don't see any visual indication that Flan dislikes the cookies so I think this is reasonable.
Adding fourth wall tag, as Flan's surprise shatters the bounds of reality.
Nh.Nh.Is it good?Yes! I did it!Wait, Flan!?Isn't that great, M'lady?At least she seems to have taken them with her.Flan...Young Mistress, it's snack time!*empty*