Gather the ingredients.

Mix cornstarch and water together in a small bowl until a paste forms.

Combine soy milk, sugar, cocoa, and vanilla in a large saucepan; stir in cornstarch paste until well combined. Cook over medium heat, stirring constantly, until boiling. Continue to cook and stir until mixture thickens, 3 to 5 minutes.

Remove from the heat and let cool for 5 minutes; pudding will continue to thicken as it cools. Cover and refrigerate until completely cool, 1 to 2 hours.