In medium saucepan, combine the sugar and cornstarch. Stir in milk. Cook and stir over moderate heat until thickened and bubbly
Remove from heat; let the mixture in the refrigerator. Add the vanilla and yogurt
Refrigerate until the mixture is cold. Melt the chocolate. While the chocolate is hot, pour it very slowly into the chilled yogurt mixture while stirring gently
Freeze in a ice-cream maker according to manufacturer's directions.
