Warm the milk in a mug for 1 min in the microwave.
In a small pot, combine the cornstarch, egg yolk, and salt. Whisk until no lumps remain. Add the hot milk, chocolate, and butter.
Heat the mixture until the chocolate melts and it begins to thicken/bubble. Remove from the heat, and pour into two ramekins. Cover with plastic wrap and place in the fridge for 2 hours.