Beat the butter with sugar until well blended.  
Mixture should be smooth and fluffy.
Blend in chocolate and vanilla.
Using an electric mixer at medium speed, beat in eggs, one at a time, taking 5 minutes to incorporate each.  *takes time but..mmmmmmmmmm sooooooo soooo good!*
Turn mixture into pie shell and chill several hours.
Garnish with whipped cream before serving.
Refrigerate any leftover pie.
Makes 8 servings.