Hershey's chocolate was like that on day 1, about 120 years ago, because they wanted to use milk without having to keep the supply refrigerated. Lipolysis of the milk fat produces butyric acid, which does taste a bit like vomit or parmesan cheese. People got addicted to the taste, so today the flavor is intentional. Some companies, targeting the American market, even add butyric acid to mimic the Hershey's taste.