In a saucepan whisk together sugar and cornstarch and whisk in milk. Bring mixture to a boil over moderate heat, stirring frequently, and boil, stirring constantly, 1 minute. Remove pan from heat.
In a small bowl stir about one fourth milk mixture into yolks and pour into remaining milk mixture, whisking constantly. Cook mixture over low heat, stirring constantly, until slightly thickened and a thermometer registers 160 degrees F. (Do not boil mixture or it will curdle.) Stir in creme de menthe and transfer mixture to a bowl. Chill mixture about 1 hour, or until cool.
In a small heavy saucepan melt chocolate over low heat, stirring occasionally, and cool slightly. Put chocolate in a small sealable plastic bag and seal bag, pressing out excess air. Squeeze chocolate to one corner of bag and snip off 1/8 inch from corner to form a makeshift pastry bag. Freeze milk mixture in an ice-cream maker and with motor running add chocolate in a thin stream during last few minutes of freezing time.
Recipe courtesy of Gourmet Magazine