Preheat oven to 300°F. Melt chocolate in double boiler, add milk, a speck of salt, and cornstarch mixed with a little cold water, and cook until smooth and thick, stirring constantly; add butter. Mix the yolks and sugar and pour the hot mixture over them, stirring well; put back in double boiler and cook, stirring constantly, until mixture is very thick. Cool, add vanilla. Beat egg whites until stiff and fold in. Fill shell with chocolate custard and bake for 30 minutes. Serve with whipped cream.