1. Place 2 cups of the milk in a medium saucepan over medium heat.
2. Remove from the heat when milk just begins to simmer.
3. Add chocolate to the hot milk and stir in until completely melted.
4. Combine remaining 1 cup of milk, sugar and cornstarch in a bowl, then stir into hot milk and chocolate mixture.
5. Return pan to medium heat and cook, stirring until mixture thickens, approximately 10 minutes.
6. Remove from heat and let it cool to room temperature.
7. Transfer to a bowl, cover and refrigerate overnight.
8. Process in an ice cream maker according to manufacturer's directions.