In mixer bowl, cream butter, gradually add sugar, creaming till light. Blend in chocolate and vanilla. Add eggs, one at a time, beating 3 minutes after each addition, on medium speed of electric mixer. Turn into pie shell. Chill several hours.
Pie shell: Use oreo cookies with cream centers scraped off (or chocolate wafer cookies, but I like oreos the best.) Combine cups crushed cookie crumbs with melted butter. Press into 20 cm or 23 cm pie plate (I use a glass one). Bake at 190ºC for 10 minutes and then let cool completely before filling (I often make the shell one day and the filling the next just cover shell with plastic wrap after it has cooled to keep it fresh till you fill it.)