In a heavy medium saucepan combine the egg yolks, sugar, milk, half-and-half or light cream, chocolate, and coffee crystals. Cook and stir over medium heat until the mixture is slightly thickened and bubbly; remove from heat. Beat mixture with a rotary beater until smooth. Transfer to a medium bowl. Cool thoroughly by placing bowl in a sink or large bowl of ice water; stir occasionally. Stir in whiskey.
In a large mixing bowl beat whipping cream with an electric mixer on medium speed until soft peaks form (tips curl). Fold in cooled chocolate mixture. Pour into an 8x8x2-inch baking pan. Cover and freeze at least 8 hours or overnight until firm. Let stand at room temperature for 10 to 15 minutes before serving. Makes about 12 servings (1-1/2 quarts).