Preheat the oven to 325ºF and bring the cream and milk to a low simmer in a small saucepan. Remove the mixture from the heat and add in the instant espresso powder and finely chopped dark chocolate and whisk until everything has melted together.
In a medium bowl, whisk the egg yolks to combine. While whisking, mix in the sugar, vanilla extract, and kosher salt. While continually whisking, slowly mix in the chocolate and cream mixture. Strain the mixture through a fine mesh sieve into a glass measuring cup and set aside.
Meanwhile, set out three 1 cup oven-safe custard dishes in a square baking pan. Divide the chocolate custard mixture amongst the dishes and cover each with aluminum foil. Pour enough very hot tap water around the custard dishes to come about halfway up the sides then carefully transfer the pan to the oven.
Bake until the custards are just set, about 45 to 50 minutes. Remove the custard dishes from the baking pan, uncover, and place in the fridge to chill for 2 to 3 hours before serving topped with a little whipped cream, if desired.
These pot de crème can be made up to 3 days in advance stored in the fridge.