Chop chocolate. In a heavy saucepan whisk together sugar and cocoa powder until combined and whisk in cream and milk. Bring mixture just to a boil, stirring occasionally. In a bowl beat yolks until smooth. Add hot cream mixture to yolks in a slow stream, whisking, and pour into pan. Cook custard over moderately low heat, stirring constantly, until a thermometer registers 170° F. Remove pan from heat and add chopped chocolate, whisking until melted. Pour custard through a sieve into a clean bowl and cool. Chill custard, its surface covered with plastic wrap, at least 3 hours, or until cold, and up to 1 day.
Transfer custard to bowl of a standing electric mixer and beat just until thick and fluffy. Freeze custard in an ice-cream maker. Transfer ice cream to an airtight container and put in freezer to harden. Ice cream may be made 1 week ahead.
