In a small bowl, whisk together ¼ cup milk and cornstarch until smooth. Set aside. In a medium saucepan, combine the remaining 1¾ cups milk, sugar, chocolate, cocoa powder, vanilla, and salt. Heat over medium-low heat, whisking, until chocolate is melted. Whisk in the cornstarch mixture until fully incorporated. Reduce the heat to low, and continue to stir briskly with a wooden spoon or a heatproof spatula. The mixture will come to a simmer and slowly begin to thicken. Continue to cook 1 to 2 minutes, or until the pudding coats the back of the spoon and slowly drips off. Remove from heat and transfer to a bowl. Cover with waxed paper directly on the surface. Chill until nice and cold in the fridge.