The obvious determining factor in this recipe is the quality of the chocolate. We seek out and use craft chocolate wherever possible. It does make a difference.
Special equipment: Ice cream maker
For the frozen chocolate custard:1.
Set the cocoa and chopped bittersweet chocolate aside in a large heatproof bowl.2.
Put the egg yolks and sugar into a heavy medium saucepan and whisk until smooth. Gradually whisk in the cream and milk. Cook over medium heat, stirring frequently with a wooden spoon, until the custard reaches a temperature of 170°F on an instant-read thermometer or is thick enough to coat the back of the spoon.3.
Pour the warm custard into the large bowl with the 2 chocolates and let it sit for a minute to melt the chocolate. Add salt and stir well, smoothly incorporating all the chocolate.4.
Strain the custard into a medium bowl. Set that bowl into a larger bowl filled with ice, then stir the custard frequently until it has cooled. Cover the custard and refrigerate it until completely chilled, about 4 hours.5.
Churn the custard in an ice cream maker following the manufacturer's instructions. Scoop the ice cream into a quart container with a lid, cover, and store in the freezer. Custard can be kept up to a month in the freezer, but we prefer to eat it within 24 hours!
For the chocolate shake:
Put the frozen chocolate custard and milk in a blender. Blend on high until smooth. Serve in a tall cold glass.