1. In a double boiler over barely simmering water, melt chocolate. Brush 6 paper muffin cups with melted chocolate and place in freezer 5 minutes. Repeat 5 times, forming 6 thin layers of chocolate on each cup. (Repeated layerings gives cups strength.) Put muffin cups on tray in freezer until frozen (1 to 3 hours), then remove and carefully place in an airtight container and return to freezer. They may be made up to 1 week ahead.

2. In a medium saucepan over moderate heat, bring milk, cream, and salt to a simmer. With an electric mixer at medium-high speed, beat yolks 3 minutes. Add sugar gradually, beating until mixture is pale and thick (5 to 7 minutes). Remove bowl from mixer and whisk in 3/4 cup of the hot-milk mixture by hand. Pour resulting mixture into the saucepan and cook over moderately low heat, stirring constantly with a wooden spoon, until mixture reaches 180 degrees F on an instant-read thermometer. Do not allow custard to boil.

3. Strain custard through a fine sieve into a bowl set over ice. Stir occasionally until completely cold. Stir in whiskey.

4. Remove coated muffin cups from freezer. Peel off paper. Spoon in custard mixture. Return cups to freezer for at least 2 hours or up to 1 day. If custard seems very firm, remove cups from freezer 5 minutes before serving.