Finely chop the dark chocolate. In a small bowl, mix a tablespoon of cornstarch with a couple of tablespoons of cold milk to create a slurry. Set both aside.

In a medium saucepan, combine the whole milk, cocoa powder, sugar, and sea salt. Whisk together and heat over medium-low for about 3-5 minutes until warm but not boiling.

Stir in the cornstarch slurry and whisk vigorously to avoid lumps. Continue to heat for about 3-5 minutes until thickened.

Lower the heat and gradually stir in the chopped dark chocolate until melted and smooth, about 2-3 minutes.

Remove from heat and pour into mugs. Top with whipped cream, grated chocolate, or dust with cocoa powder.