Chocolate Frozen Yogurt

Stir together the cornstarch and sugar, and place in a saucepan. Stir in the evaporated milk, and chocolate chips. Cook over medium heat until chocolate is melted and the mixture is thickened. Stir constantly to prevent burning.
Remove the mixture from the heat, and stir in the plain yogurt and vanilla. Refrigerate until chilled.
Pour the mixture into an ice cream maker, and freeze according to the manufacturer's instructions.