Mix 1/4 cup of the milk with the cornstarch and set aside for later use.
Put 1 cup milk, cocoa powder, sugar, vanilla and salt in a saucepan on medium heat.
Whisk until sugar is dissolved and mixture starts bubbling. Add the milk-cornstarch mixture to the saucepan and keep whisking until it thickens (5-6 mins).
Take off heat and leave it to cool. It will keep on thickening while cooling. You can keep it in an airtight container in the fridge for up to 5 days.