The exact recipe for Oreo cookie filling is an industry secret held closely by Nabisco, the manufacturer of the cookie. Recipe expert and cookbook author Gloria Pitzer's copycat Oreo cookie filling recipe is made with unflavored gelatin, shortening, vanilla extract, powdered sugar and cold water.
Pitzer makes her version of Oreo cookie filling by softening the gelatin in the cold water, then placing the dissolved gelatin in a heatproof cup in a bowl of hot water until it becomes clear. The shortening is beaten until soft, then combined with the sugar and vanilla extract and beaten for about 10 minutes, or until well-incorporated. When the gelatin has cooled completely, it is added to the shortening mixture until ready to use.