In large mixing bowl blend together sugar, cornstarch and salt. Gradually stir in milk, mixing well. Add chocolate squares. Microwave on HIGH 5 to 7 minutes, stirring every 3 minutes until mixture is smooth, thickened and clear.
Stir a small amount of hot pudding quickly into egg. Return egg mixture to hot pudding, mixing well. Microwave on HIGH 1 to 3 minutes, stirring after 1 minute, until smooth and thickened.
Add butter and vanilla. Stir until butter is melted. Pour into serving dishes.
If you are not going to serve immediately, place plastic wrap on the top of pudding so it won't form a film.