Place dark chocolate in a medium heatproof bowl and set aside.

In a small saucepan, whisk together sugar and corn starch so there are no lumps. Gradually whisk in about half of the milk until smooth. Whisk in remaining milk and then place over medium-low heat and whisk constantly until the mixture reaches a gentle simmer and some bubbles appear at the surface, about 8-10 minutes.

Whisk egg well in a small bowl and then very gradually add some of the hot thick milk mixture while whisking constantly. Add about half of the mixture. Then, off the heat, gradually whisk this egg mixture back into the milk mixture in the saucepan until smooth. Return to low heat and bring to a boil while whisking. Once at a boil, cook for 1 minute.

Pour the hot mixture into a bowl with the dark chocolate. Let stand for a minute and then stir until smooth and the chocolate is melted. Stir in vanilla and salt and a bit of butter if you wish for extra lush. Serve warm, or place plastic wrap directly on the surface of the pudding and refrigerate until set, about 1 hour.