French Silk Pie

This is my BIL's favorite pie. :) This is from allrecipes but I altered it slightly.

2 sticks butter
1 1/2 cups white sugar
4 (1 ounce) squares unsweetened baking chocolate
2 teaspoons vanilla extract
4 eggs
1 (9 inch) pie shell, baked

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.

I top with shaved chocolate (white & dark) and/or fresh whipped cream!