In small bowl, combine cornstarch and water to form a paste.
In large saucepan over medium heat, stir together soy milk, vanilla, sugar, cocoa and cornstarch mixture. Cook, stirring constantly, until mixture boils. Continue to cook and stir until mixture thickens. Remove from heat. Pudding will continue to thicken as it cools. Allow to cool five minutes, then chill in refrigerator until completely cool.