Step 1

In a medium saucepan, combine sugar and cornstarch or flour. Gradually stir in milk and unsweetened chocolate.

Step 2

Cook and stir mixture over medium-high heat until mixture is thickened and bubbly. Reduce heat; cook and stir for 2 minutes more. Remove from heat.

Step 3

Separate egg yolks from whites; dispose of egg whites or save them to make a meringue. Beat egg yolks lightly with a fork. Gradually stir about 1 cup of the hot filling into yolks.

Step 4

Return all to saucepan; bring to a gentle boil. Cook and stir for 2 minutes more. Remove from heat.

Step 5

Stir in margarine or butter and vanilla.

Step 6

Pour the hot filling into the baked pastry shell. If making a meringue, chill pie in refrigerator for 1 hour before topping with meringue. Then return pie to refrigerator.

Step 7

Chill pie in the refrigerator for at least 12 hours. If you didn't make a meringue, top with Cool Whip or whipped cream before serving.