Heat the oven to 400° F (205° C).
Using a fork, prick the bottom and sides of the pie crust. Bake for about 10-11 minutes or until slightly golden brown. Let it cool before adding the filling.
Meanwhile, prepare the filling. Add chocolate and butter to a microwave-safe bowl and heat for about 60-90 seconds or until melted. Stir well until smooth and reserve.
In a large bowl, add the sugar, flour, salt, cocoa powder, milk, vanilla, and eggs. Beat for about 2 minutes with an electric mixer, or until the mixture is homogeneous and bubbly. Add the reserved melted chocolate mixture and beat for about 1 minute or until smooth.
Pour it into the pie crust and reduce the oven temperature to 350° F (180° C).
Place the filled pie crust onto a baking sheet and bake on the middle rack for about 35-45 minutes or until the top is set. It will have a hard crackly top with a fudgy filling underneath. Make sure the center top feels somewhat firm when touched before removing it from the oven.
If the pie crust appears to be browning too much, place a foil ring around the edge of the pie crust.
Let the pie cool completely over a rack. Chill in the fridge for a few hours before slicing. This will allow the filling to set and harden.