1. Mix cornstarch and sugar in a medium saucepan. Slowly drizzle in 1/2 cup milk, whisking until smooth, then whisk in remaining 2 cups milk. Turn heat to medium-high and bring to a boil. Lower heat to medium and whisk until thickened, 3 to 4 minutes.
2. Remove from heat and add chocolate chips, butter and vanilla. Whisk until smooth.
3. Divide pudding into 8 parfait glasses. Serve warm, or press plastic wrap onto surface of each pudding and chill.