In a medium saucepan, combine the brown sugar, cocoa powder, cornstarch and salt. Whisk to combine. Add the milk and continue to whisk. Place over medium low heat and cook until thickened, about 10 minutes. After 10 minutes, bring the mixture to a simmer and then reduce the heat to low, and cook for 2 minutes while whisking until very thick. Remove the pan from the heat and add the chocolate, butter and vanilla. Stir to combine. Transfer pudding to the cooled pie shell and refrigerate for 2 hours.