Mix milk, cornstarch, sugar, raisins and vanilla extract (if used) in a small saucepan over medium heat. Cook it until a dough forms, stirring constantly.
Scoop about a teaspoon of the dough and roll it between your hands, forming a ball; keep making balls.
Put them on a serving plate and sprinkle the cocoa powder on top.
1 tsp Cocoa Powder
Notes
The time that it takes for the mixture to thicken in the saucepan is different for everyone. You just have to trust the process and keep stirring until a dough-like consistency forms.