Combine egg yolks, 3/4 cup sugar, flour, chocolate, and butter in top of a double boiler; mix well. Gradually add milk, stirring well. Cook over boiling water, stirring constantly, 20 minutes or until thickened. Stir in vanilla and salt. Pour filling into pastry shell.
Beat egg whites (at room temperature) and cream of tartar until foamy. Gradually add 1/4 cup plus 2 tablespoons sugar, 1 tablespoon at a time, beating until stiff peaks form. Spread meringue over filling, sealing to edge of pastry. Bake at 425° for 6 minutes or until meringue is lightly browned. Cool.