01 -
In a small bowl, whisk the cornstarch, cocoa powder, sugar, salt, and 1/2 cup of the cold milk until completely smooth.
02 -
Pour the remaining 1 1/2 cups milk into a small heavy saucepan. Warm over medium-low heat until steaming around the edges, about 2 minutes.
03 -
Slowly whisk the cocoa slurry into the warm milk. Cook for 2 to 3 minutes, whisking constantly, until the mixture begins to thicken.
04 -
Add the chopped dark chocolate and keep whisking over low heat until melted, glossy, and smooth, about 1 to 2 minutes.
05 -
Remove from the heat when the hot chocolate coats a spoon but still pours. Stir in the vanilla, pour into 2 small cups, and top with whipped cream if desired.