Preheat oven to 325°F.
Place pie shell in oven. Bake for 8 - 10 minutes, until the bottom of shell is just starting to brown. Remove from oven and set aside to cool.
In a large mixing bowl, add flour, sugar, salt and cocoa. Mix well with a whisk.
Add milk. Mix well with a spoon or electric mixer.
Add eggs, one at a time, mixing between each.
Add vanilla and butter. Beat until smooth.
Pour batter into the pie shell.
Place pie in preheated oven. Bake for 35 to 40 minutes, until a knife inserted in the center comes out clean.
Remove from oven. Place on a wire rack to cool.
When cooled, refrigerate, covered, for 3 hours or until well-chilled.
If topping with whipped cream, make sure that the pie has been completely cooled before topping.