In a medium saucepan, combine the milk, evaporated milk, and vanilla and heat until just barely simmering.

Meanwhile, in a small bowl combine the cocoa, sugar, cornstarch, coffee powder (if using), and salt.

When the milk mixture is simmering, carefully pour about 1/4 cup into the cocoa mixture and whisk to form a paste. Whisk the paste back into the saucepan with the milk. Heat over medium heat until the hot chocolate is just barely starting to boil and you see one or two bubbles pop to the surface.

Remove from the heat, pour into mugs, and serve, or store in an airtight container in the fridge for about a week.