Make the Caramel
Place 5 (6 oz) ramekins in a 9 x 13-inch baking dish and set aside.
Place the sugar and water in a small saucepan and allow the sugar to dissolve over low heat.
Bring to a simmer and let it gently bubble away until it reaches a rich amber color, roughly 8 to 10 minutes.
Immediately remove from the heat and divide evenly among the prepared ramekins. Set aside to set.
Make the Custard
Add the milk into a medium saucepan and bring to a simmer over medium heat, about 3 minutes.
Once the milk comes to a simmer, turn off the heat and add in the chocolate. Let the mix stand for 5 minutes for the milk to melt the chocolate, then whisk until smooth.
Meanwhile, in a medium bowl, whisk the whole eggs, yolks and sugar together until smooth.
Stir in the rum (if using) and the salt.
While whisking constantly, gradually pour the hot milk mixture into the eggs. Keep whisking until fully blended.
Pour the custard through a sieve and into a glass measuring jug with a pour spout. Set aside to cool down completely or cover and place in the fridge for up to 2 days.
Bake the Custard
Preheat the oven to 300°F (150°C) and fill a kettle with water and bring to a boil.
Divide the custard evenly among the prepared ramekins.
Place the baking dish on a middle rack in the oven and pour enough boiling water from the kettle to come halfway up the sides of the ramekins.
Bake for 30 to 35 minutes, until the edges of the custards are set but still a bit jiggly in the center.
Remove from the water bath and let cool to room temperature before transferring to the refrigerator to cool completely, at least 4 hours but preferably overnight.
When ready to serve, carefully run a thin-edged knife around the edge of the custards and invert them onto individual serving plates. Serve as is with the lovely caramel sauce on top. Enjoy!