Preheat oven to 325 degrees F. Position a rack in the center of the oven.
Place the chocolate chips in a double boiler and melt over low heat, stirring constantly.
In a medium bowl, whisk together the sugar, flour, and salt until well combined. In a large mixing bowl, combine the eggs, buttermilk, and vanilla. Add the sugar mixture and mix with an electric hand mixer or whisk vigorously. With a rubber spatula, stir the melted chocolate into the batter.
Pour batter into the piecrust; you will have about 1 cup of left-over batter. Place pie in oven on middle rack. Bake for 1 hour and 15 minutes to 1 hour and 25 minutes, or until the pie is crisp on top and a knife inserted in the center comes out with just a bit of moist chocolate on it.
Remove from oven and cool completely. Let stand at least 1 hour before serving. If not eating immediately, refrigerate pie. This pie can be served warm or chilled. Garnish with store-bought whipped cream just before serving.