Peanut butter — Food safety facts
Feed with care · Cats · full breakdown on the linked food page
Safe only if xylitol-free (many brands now add it — xylitol is fatal to dogs). High fat and salt — small amounts.
Short answer
A tiny lick is likely not dangerous, but peanut butter is not an ideal treat for cats. The primary risk for dogs is xylitol, a sweetener that is fatal to them, so you must always check the label. For cats, the concern is the high fat and salt content, which can lead to digestive upset or contribute to obesity if given regularly. If your cat seems fine after the single lick, just monitor it; but if you notice vomiting or diarrhea, a call to your vet is wise.