Whisk together milk, sugar, cocoa, cornstarch, instant espresso, and salt in a medium-size heavy saucepan over medium-high heat, and cook, whisking constantly, 5 minutes or until mixture is hot. Gradually whisk 1/3 cup hot milk mixture into egg. Whisk egg mixture into remaining hot milk mixture.
Cook, whisking constantly, 3 minutes or until mixture thickens. Remove from heat, and add chopped chocolate, stirring until chocolate melts and mixture is smooth. Stir in vanilla. Pour mixture into a glass bowl. Place heavy-duty plastic wrap directly on warm mixture (to prevent a film from forming), and chill 2 hours or until pudding is completely cool.
Spoon 1/2 cup pudding into each of 5 individual serving dishes, and top each with 1 Tbsp. whipped topping. Garnish, if desired.