Prepare a large bowl filled with ice and cold water and set aside.
In a medium bowl, beat the egg yolks with 6 tablespoons of sugar until you get thick ribbons, about 2 1/2 minutes. Set aside.
Meanwhile, set the chocolate in a glass or metal bowl over a medium heavy saucepot filled a few inches with water. Heat on medium until simmering, but not boiling. Stir the chocolate until melted and smooth. Remove from heat and set aside.
In a small saucepan, add the milk and cream with the cocoa powder. Whisk until fully combined and increase the heat to medium. Once it starts to boil around the edges, remove from the heat and set aside.
Slowly whisk half of the hot milk mixture into the melted chocolate, a few additions at a time. At first, it will look like you curdled the chocolate- that’s good. Keep stirring. After a few more additions, it will smooth back out again. Set aside.
With the other half of the milk, slowly whisk it into the egg yolks, just a few tablespoons at a time.
Slowly add the chocolate mixture to the egg mixture, whisking constantly.
Once fully combined, return everything to the medium saucepot. Switch to a wooden spoon, and stir gently over low heat until a thermometer shows 175 F, around 4-5 minutes. Make sure to go slow and simply just wait.
Quickly transfer the custard to a bowl and set that bowl over the ice-filled bowl. Stir with the wooden spoon until it starts to cool down. Allow it to continue to cool over ice.
Rinse out small saucepot and return it to the medium high heat with the 6 remaining tablespoons of sugar and 2 tablespoons of water. Swirl it until the sugar dissolves, and if needed, brush down the sides of the pan with a pastry brush dipped in water if you see crystals forming. Do not stir the sugar! Once the sugar reaches a moderate amber colour and the thermometer reads 350 F, take it off the heat.
Slowly whisk the additional 1/3 cup of cream and into the sugar mixture and then remove from heat. Stir in vanilla.
Whisk the caramel into the chocolate custard, then you can strain through a fine sieve into a flat container. Cover and chill for at least 5 hours to over night.