No, cats can't have chocolate. I would suggest not to give them any just to be on the safe side.
Chocolate can contain high amounts of fat and caffeine-like stimulants known as methylxanthines. If ingested in significant amounts, chocolate can potentially produce clinical effects in cats ranging from vomiting and diarrhea to panting, excessive thirst and urination, hyperactivity, abnormal heart rhythm, tremors, seizures and even death in severe cases.
However white chocolate has a significantly lower amount of methylxanthines, which makes it less dangerous. So, if your cat ate some white chocolate, I wouldn't be worried as long as it wasn't a very large amount, but I would check with a vet to be sure. Small amounts may only cause an upset stomach.