In a saucepan, combine the heavy cream, whole milk, and half of the sugar. Heat over medium heat until it begins to steam, but not boil.
In a separate bowl, whisk the egg yolks with the remaining sugar until smooth.
Gradually pour a small amount of the warm cream mixture into the egg yolks to temper them, whisking constantly. Slowly add the tempered egg yolks back into the saucepan.
Cook the mixture over medium-low heat, stirring constantly, until it thickens and coats the back of a spoon (about 170°F / 77°C).
Remove from heat and stir in the chopped dark chocolate, cocoa powder, and vanilla extract until smooth.
Let the custard cool to room temperature, then refrigerate until fully chilled (about 2-3 hours).