In pan stir together sugar and cornstarch. Slowly add milk and stir until well mixed. Add the chocolate. Cook over medium-low heat stirring constantly. Once it thickens and becomes bubbly cook and stir for 3-4 minutes longer.
Remove from heat. Gradually wisk about 1/2 to 1 C of the hot mixture into the beaten egg. Stir egg mixtrue into pan.
Return pan to heat, cook and stir an additional 2 minutes.
Remove from heat. Stir in butter and vanilla until butter is melted. Place in custard cups and chill in refridgerator. (if you can get that far...........I usually end up eating it hot before it ever gets to the fridge!!!)
For an extra yummy treat, top hot pudding with a scoop of vanilla ice cream.