Add Sugar, Cornstarch, chocolate and Salt to medium saucepan (2-3 quart) and whisk to combine.
Add Milk and whisk well to combine.
Over medium heat, bring the mixture to a boil while whisking constantly.
Once the mixture starts boiling, boil for 2 minutes. The mixture will thicken.
Remove from heat.
Blend a few spoons of the hot mixture into egg yolks to temper the yolks.
Add the tempered yolks to the mixture, blending well and continue to cook for a minute or two just until the mixture starts to bubble.
Remove from the heat and pass through a fine mesh strainer into a large bowl. This will make for a smoother pudding and remove any potential bits of egg that may have cooked.
Add softened butter and vanilla to the mix and pour into prepared pie crust.
Cover with plastic wrap directly on the pudding and refrigerate for about 3 hours or until set.
Serve with whipped cream if desired.