In one medium mixing bowl, place all 6 egg yolks. In a separate mixing bowl, melt the chocolate.
Pour the milk and heavy cream into a heavy bottomed sauce pot and heat over medium heat until just about to simmer.
Just before the milk and cream are hot, whisk the granulated sugar into the egg yolks until combined and smooth.
Place the bowl with egg yolks and sugar on a damp rag, take the pot with hot cream and milk off the heat.
Use a ladle to slowly add the milk and cream into the egg yolks while whisking constantly.
Continue adding the hot milk and cream and keep whisking the egg yolks until about half of the milk and cream have been added. This is called tempering the eggs.
Slowly ladle the tempered eggs back into the remaining cream and milk while constantly whisking the milk and cream.
Once all the tempered eggs have been added back in, return the pot to medium heat. Switch to a rubber spatula and stir the custard constantly while heating. Gently stir in a figure eight pattern and then around the edges of the pot.
Continue cooking until the custard has thickened, all of the bubbles from the surface will have disappeared.
Remove from the heat and ladle half of the hot custard through a sieve over the melted chocolate. Ladle the other half through a sieve into a new bowl.
Whisk the chocolate into the custard until it is smooth and no chocolate bits remain.
Cool both custards either in an ice bath (made with ice and water in a tray that the bowl is then set into) or cover the bowls and place them in the fridge until cold.