Add the heavy cream and milk to a medium-sized saucepan set over medium heat. When the milk begins to steam and small bubbles form around the edge of the pan, remove it from the heat.
Add the chopped chocolate to the cream and whisk until the chocolate is fully melted.
Add the egg yolks, sugar, vanilla, and instant espresso powder (if using) to a large heatproof bowl. Whisk until the egg yolks and sugar are well combined.
Slowly pour the chocolate-cream mixture into the egg yolks, whisking constantly. If you add all of the hot cream at once, the egg yolks will scramble so make sure to add slowly.
Use a ladle to pour the mixture into four ramekins. Place in a 9 x 13 cake pan and fill halfway with hot water. Cover the pan with foil.
Bake for 30-45 minutes (baking time will also depend on the size of ramekin). Bake until the centers are set but still slightly jiggly.
Remove from the oven and allow the pots to cool to room temperature. Then, transfer them to the fridge to chill for at least two hours.