Ok, I’ve found it. Siwon Lassi Shop, Ck. 26/16-A, Kachouri Gali, Chowk, Varanasi. I has the apple lassi. Today I’m going back for the fruit lassi (apple, pomegranate, and whatever else is in season). Pappu is a master. Images speak for themselves.
On another note, through a friend, I’ve also found a silk guy, so if you come to Varanasi and want to buy what you know is good quality, beautiful silk (otherwise there is a major risk of being duped), see Arun at Banaras Art Emporium, +91 9839 268 686 or firstname.lastname@example.org (explain who sent you). He owns a factory so it isn’t a shop, but a room in his house with cupboards of high quality fabric. He can make anything and will ship. I spent small fortune on silk bedding (there is a tailor who will put together whatever you like).