Preheat oven to 350 degrees. Place the prepared pie dough on a lightly floured work surface. Roll out to fit your pie pan. Crimp the edges and prick the dough with a fork. Refrigerate for 30 minutes.
Line the pie crust with foil or parchment paper. Add dried beans or pie weights to the pie and bake for 15 minutes. Remove the foil and beans and bake for 5 minutes longer, until the crust is dry but not browned.
While the crust is baking, prepare the filling. In a large mixing bowl whisk together the sugar, cocoa powder, butter, eggs, evaporated milk, vanilla and salt until smooth.
Pour the filling into the pre-baked pie shell and bake for 45 minutes, or until the filling is set around the edges but a little jiggly in the center. Cover the crust with strips of foil halfway through baking to prevent over-browning. Cool the pie on a rack before cutting. Serve with whipped cream.