Steps
1. In a small saucepan, combine milk, cream, and sugar over medium heat; stir until sugar dissolves.
2. Add butter and chopped dark chocolate; stir continuously until melted and smooth.
3. In a small bowl, mix cornstarch with a tablespoon of cold milk to form a slurry.
4. Slowly add the slurry to the saucepan while stirring.
5. Add cocoa powder, vanilla extract, and a pinch of salt; keep stirring until the mixture thickens enough to coat the back of a spoon.
6. Remove from heat and pour into a serving cup.
7. Let it cool for 10 minutes at room temperature, then refrigerate for at least 1 hour before serving.
8. Optionally, garnish with chocolate shavings or a small pat of butter for extra richness.