Bring the eggs to room temperature if you haven't already.
Combine the chocolate and butter in a microwave safe bowl. Heat on high for 30 seconds. Stir well. Repeat until chocolate is melted and mixture is smooth. Alternatively, you can do this in a double boiler.
In a large bowl, whisk together the sugar, eggs, yolk, salt and vanilla. Add the slightly cooled chocolate mixture to the egg mixture, then scrape into the partially baked pie shell.
Set pie on a rimmed baking sheet and bake for 45 minutes or until the center is set. Let cool to room temperature.
Once cool you can garnish it with confectioners' sugar. We like laying damp strips of parchment over the top, sprinkling powdered sugar around the strips, and making a grid pattern.