Melt the chocolate and mix it with 1/4 liter of the milk (using a whisk). Stir the chocolate powder and cornstarch into the chocolate milk.
Meanwhile, bring the rest of the milk to the boil. When it boils, stir in the chocolate milk and bring to the boil again and sweeten with sweetener or sugar as desired.
Pour the pudding into bowls, let it cool down and maybe serve with a little cream.