In a small saucepan, whisk together the sugar, cornstarch, and salt. Set aside. In a small bowl, whisk together the egg yolk, whole milk, 1/4 cup heavy cream, and vanilla extract. Slowly whisk the wet ingredients into the dry until completely incorporated.
Cook the mixture over medium heat, whisk very frequently, until it becomes very thick. This will take about 10 minutes. Once the mixture starts to thicken, you want to whisk it constantly.
Remove the mixture from the heat, and immediately whisk in the dark chocolate chips and butter until completely smooth. Press the mixture through a fine mesh siv to remove any lumps that formed during the cooking process. Press a sheet of plastic wrap directly on top of the custard mixture, and chill for about 2 hours, or until it is completely chilled through.
Once the custard is chilled, beat the remaining heavy cream until you reach soft peaks. Beat in the powdered sugar. Do not overbeat- soft peaks are the perfect stopping point here! Scoop a large spoonful of the whipped cream into the custard, and fold it in gently. Repeat until all of the whipped cream has been added. Spoon into your serving bowls, and top with more whipped cream if you'd like. Enjoy!