The Pasticceria Mattei is a Prato institution and the original source of what the rest of the world calls "biscotti." The proper Pratese name is cantucci, and the definitive version — dry, crunchy, full of whole almonds, meant to be dunked in vin santo — has been made to the same recipe here since 1858. The little blue paper bag is unmistakable.
The shop is on Via Ricasoli, a few minutes from the cathedral. You can buy cantucci to take away, and sometimes a glass of vin santo to dunk them in on the premises. They also make other traditional sweets: brutti ma buoni, ricciarelli, and seasonal pastries. This is not a tourist trap; it is simply the right place to buy this particular thing.