Place a rubber spatula and a whisk on a plate next to the stovetop to have them ready to use interchangeably.
Prepare the chocolate: place the chopped chocolate into a large heatproof bowl and set the bowl near the stovetop.
Prepare the egg yolks: put the egg yolks (4 egg yolks) in a medium heatproof bowl, and whisk them lightly to break them down; set the bowl near the stovetop.
Warm the milk and the sugar: place the milk (475 g; 16.8 oz) and the sugar in a medium saucepan. Warm over medium heat, often stirring, until the sugar dissolves.
Temper the egg yolks: when all the sugar dissolves and the milk is hot and steamy (this is at 75ºC / 167 ºF if you have a thermometer), remove the saucepan from the heat and slowly ladle roughly half of the warm milk over the egg yolks with one hand while whisking them vigorously with the other hand to temper them.
Cook until thickened: pour the tempered yolks & milk back into the saucepan and over medium-high heat. Cook, constantly stirring with a rubber spatula and scraping the bottom of the saucepan to prevent the custard from scalding. Maintain a medium temperature, do not let it come to a boil.
Remove from the heat when the custard starts to thicken (82ºC / 179 ºF / when it thickens to coat the back of a spoon / when you tilt the saucepan, a layer of thickened custard appears to form on the bottom).
Pour the custard over the chopped chocolate. Let it sit for 1 minute.
Whisk to melt the chocolate, scraping with the rubber spatula the insides of the bowl as needed.
Add the heavy cream (355 g; 12.5 oz) and stir to combine.
Blend with an immersion/regular blender until it is a uniform brown colour with no streaks, pausing to scrape the bottom and sides of the saucepan/blender jug with the rubber spatula as needed.