Preheat oven to 350°F.
Mix together butter, vanilla and brown sugar until well mixed.
Add in eggs one at a time, mixing well between additions.
Stir together cocoa powder, flour, and salt and add to wet ingredients.
Mix until combined but do not overmix.
Pour filling into prepared pie crust.
Bake for 35-45 minutes - the center will still be a little jiggly.
Cool for at least an hour, serve either warm or at room temperature with vanilla ice cream or whipped cream.