Melt chocolate. Whisk together 1/2 cup milk & cornstarch. In a small saucepan, bring remaining 1 1/2 cups milk, sugar, cocoa & salt just to a simmer over medium heat. Add cornstarch/milk mixture; bring to a boil, whisking constantly. Cook 1 minute. Remove from heat & stir in melted chocolate & vanilla. Spoon into serving glasses. Cover surface directly with plastic wrap to prevent skin from forming. Chill.