In a bowl, beat together the egg yolks, cornstarch and salt. In the top of a double boiler, add the milk and sugar, heat until hot. Pour about a third of the milk into the egg mixture and mix. Pour this into the double boiler and cook until thickened. Remove from heat and stir in the butter and vanilla. Stir in the chocolate until melted. Refrigerate until cool. Pour the pastry cream into the prepared pie crust, top with whipped cream and grated chocolate.