toasted hazelnut paste with chocolate
The story goes that during the Napoleonic blockades, getting cocoa to Europe from South America was almost impossible. To extend what cocoa was available, people began to add hazelnuts, a technique perfected in mid 19th-century Piedmont, using a paste of toasted hazelnuts. It was named gianduja after Gianduja, the archetypical 18th-century Piedmontese figure carnival mask.
It's a popular gelato flavour and is also used to make Gianduiotti as well as Nutella.