Melt unsweetened chocolate squares in the microwave. Set aside to cool.
In the bowl of an electric mixer, cream together butter and sugar until light in color and fluffy.
Mix in melted, cooled chocolate and vanilla.
Add eggs, 1 at a time, beating at least 5 minutes between each addition and scraping sides of the bowl to incorporate all the sugar. (Do not underbeat...this step is essential to a light in texture French Silk Pie).
Continue beating 5 additional minutes.
Spoon and smooth into pre-made graham cracker crust.
Chill until set, about 1 hour.
Serve topped with Cool Whip and chocolate shavings.