In medium sauce pan add ¾ cup milk, heavy cream and sugar and heat over medium heat until sugar dissolves and it starts to bubble around the edges (about 7-8 minutes).
In a small container whisk together your remaining ¼ cup milk and cornstarch together.
Once your milk, cream and sugar mixture has started bubbling whisk in your cornstarch mixture and heat again for about 1 minute.
Add in your chopped chocolate and whisk over medium heat until chocolate melts and it thickens (about 2-3 minutes).
Whisk in your vanilla extract (this is optional) until combined.