New Delhi, June 23 -- In a scary world, comfort food can be a source of solace. Tasty Congee & Noodle Wantun Shop, a restaurant in Hong Kong, offers soothing rice porridge topped with scallions to locals, visitors from mainland China and any foreigners who have acquired the taste. The bill can be paid in Hong Kong dollars or the mainland's currency, the yuan. The congee shop will even take the e-CNY, a digital currency issued by China's central bank (although the maître d' had never seen anyone use it before). Over 5,000 merchants in the city accept this novel form of money.

China's leaders seem newly optimistic about the yuan's prospects beyond the mainland. They have long been keen to ease their dependence on the American dollar f...