So, Hershey is tight lipped about this, partially because recipes, but also because openly admitting you sour your milk intentionally is bad PR. The speculatio

So, Hershey is tight lipped about this, partially because recipes, but also because openly admitting you sour your milk intentionally is bad PR. The speculation is that during WWII, the milk supply wasn’t particularly good due to war rationing so this was naturally what happened, and then after the war it turned out American consumer preference had gotten used to and expected this from Hershey’s. However, this isn’t good if, say, you’re a British expat in the US, you expect British Cadbury chocolate and get the Hershey-licensed sour stuff instead.