Combine the sugar, cocoa powder, cornstarch and salt and place in a saucepan over medium heat.
While whisking constantly, very slowly pour the oat milk into the saucepan until everything is combined and smooth (if there are lumps after this stage, there will be lumps in the final pudding).
Continue to stir constantly for approximately 10 minutes, you will see the mixture start to thicken and bubble. When the mixture thickly coats your spatula add the dark chocolate and stir vigorously until completely melted and smooth.
Take the saucepan off the heat and continue stirring as you add in the vanilla.
Enjoy while warm or chill in the fridge and serve cold, garnish with your fruit of choice!