Blend together the flour, cornstarch, salt and sugar. Add chocolate and milk. Cook until thickened, stirring constantly. Add slightly beaten egg yolks. To prevent lumping, first blend a little of the hot mixture with the egg yolks and turn back into saucepan. Cook for 1 minute longer. Add vanilla. Pour into 8- or 9-inch pie shell. Cover with meringue. Bake in 325° oven until meringue browns.