Preheat the oven to 325°F. Place six ramekins in a large baking dish or roasting pan.

In a heavy-bottomed medium saucepan over medium-low heat, whisk together the milk, heavy cream, and chopped chocolate. Cook until the chocolate is fully melted and the mixture is warm.

In a medium bowl, whisk together the egg yolks, sugar and salt until smooth. While whisking constantly, ladle in about ½ cup of the warm milk mixture to temper the eggs. Continue ladling in another 1–2 scoops of warm milk and whisk until smooth.

Pour the tempered egg mixture into the saucepan with the remaining chocolate mixture. Whisk in the espresso powder. Continue to whisk until the custard thickens slightly and coats the back of a spoon. Remove from the heat and stir in the vanilla.

Pour the custard evenly into the ramekins. Carefully place the baking dish on the oven rack, then pour water into the baking dish until it reaches halfway up the sides of the ramekins.

Bake for 50–60 minutes, until the centers are just set but still slightly wobbly when gently shaken.

Remove the ramekins from the water bath and transfer to a wire rack to cool completely. Refrigerate for at least 2 hours, or until fully chilled.