In a large bowl, whisk together the sugar, cocoa powder, and flour. Add in the beaten eggs, melted butter, vanilla extract, and buttermilk. Stir well to combine.
Pour the mixture into the unbaked pie shell being cautious not to over-fill. Bake for 40 to 50 minutes or until a crust has formed completely across the top of the pie and is mostly set. There will still be some jiggle to the pie. Allow to cool completely before slicing and serving.