One chocolate weeto probably won't be deadly since there isn't a lot of actual chocolate in them, hamsters do have some tolerance for chocolate so it isn't instantly deadly (the theobromine in chocolate is actually poisonous to all animals including humans, dogs just have a particularly low tolerance for it) but you don't want to take a risk that there is just enough chocolate in something to do real damage. So no more chocolate weetos ever and keep an eye on your ham for signs of distress. Theobromine can cause heart attacks and fits so you're watching for difficulty breathing or walking.
Always check before feeding them a new food, bookmark the
safe food list or print one out and have it handy.