You buy the authentic Italian Product - and flow the directions on the back. Franzese Italian Hot Chocolate - by Franzese Cafe Italiano at Nordstrom or on Amazon.
http://www.amazon.com/s?ie=UTF8&page=1&rh=n%3A16310101%2Cp_4%3AFranzese%20Cafe%20Italiano
Here is an interesting article and info on to make it. The writer is probably right in the article it say's they use either flour or cornstarch I guess potato flakes could also be used:
http://www.italymag.co.uk/italy-featured/guarda/cioccolata-calda-hot-chocolate-italy
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http://www.amazon.com/s?ie=UTF8&page=1&rh=n%3A16310101%2Cp_4%3AFranzese%20Cafe%20Italiano
http://www.italymag.co.uk/italy-featured/guarda/cioccolata-calda-hot-chocolate-italy