It's becoming more widely known that chocolate is very toxic to both cats and dogs. Theobromine is the offending substance here. Caffeine and other stimulants, including theobromine (found in chocolate), can poison cats. Do not feed your cat these "people foods" or leave them out where the cat could reach them. It is wise to just feed cats a commercially prepared cat food and never feed them foods meant for humans, especially "sweets."