1. Put cream and milk into a medium saucepan over medium-high heat. Heat the mixture until hot (do not let simmer), about 2 minutes. Remove mixture from heat; cover, and let stand 30 minutes.
2. Prepare an ice-water bath; set aside. Put yolks, sugar, and salt into the bowl of an electric mixer fitted with the whisk attachment. Beat on high speed until yolk mixture has tripled in volume and can hold a ribbon on surface for 2 seconds, about 3 minutes.
3. Reduce speed to medium. Ladle 1 cup cream mixture in a slow stream into yolk mixture to temper it. Add another cup cream mixture; beat to combine. Transfer mixture to saucepan; cook over medium-high heat, stirring constantly, until it is thick enough to coat the back of a wooden spoon and an instant-read thermometer registers 180 degrees;, 5 to 7 minutes. Stir bittersweet and milk chocolates into custard.
4. Pour custard through a medium-mesh sieve into a bowl set in ice-water bath. Let cool completely, stirring often.
5. Freeze custard in an ice cream maker according to manufacturer's instructions. Serve immediately, or freeze in an airtight plastic container until ready to serve. If frozen for more than 4 hours, let ice cream stand at room temperature 15 to 20 minutes before serving.

Yield: 12 servings