The day before you plan to serve, pour 4 cups of the root beer into ice cube trays to a depth of no more than 1/2-inch. Keep the remaining root beer refrigerated until ready to serve. Freeze overnight, along with tall, thin glasses or parfait glasses for serving. When ready to serve, unmold the cubes into a food processor fitted with the metal blade. Process, pulsing, just until crushed. Place a straw in each glass, and layer the parfaits by alternating scoopfuls of the root beer granita and vanilla ice cream, ending with a scoop of ice cream on the top. Serve with a pitcher of the remaining root beer, and pour root beer up to the top of each parfait at the table.
Courtesy Gale Gand, “Butter Sugar Flour Eggs” by Gale Gand, Rick Tramonto, Julia Moskin, Clarkson N. Potter Publishers, 1999