In a small saucepan, whisk together ¾ cup milk, cream, and sugar. Set the saucepan over medium-low heat and cook until it starts to bubble around the edges. While milk mixture is heating, whisk together remaining ¼ milk and cornstarch until smooth. Once the milk mixture starts bubbling around the edges, add milk and cornstarch mixture and whisk until it’s heated through, about 1 minute.

Add chopped chocolate and whisk together until smooth and thick enough to coat a spoon, about 5 minutes. Remove from heat, pour into mugs, and serve with whipped cream if desired. The drink will continue to thicken if it is left to sit off the heat.