In the top of a double boiler, heat chocolate, stirring occasionally, until chocolate is melted and smooth. Remove from heat and allow to cool to lukewarm.
In a large bowl, cream butter and sugar until pale yellow and very fluffy. Blend in the melted chocolate and vanilla. With an electric mixer on medium speed, beat in the eggs, one at a time, taking 5 minutes for each egg. Pour filling into baked pie shell. Refrigerate 4 hours before serving.