Melt the chocolate and butter together; set aside.
Beat the egg whites until foamy, gradually add 1/2 cup sugar
(still beating), then beat to soft peaks; set aside.
Beat the egg yolks, gradually adding the remaining sugar. Continue
beating until the mixture has about tripled in volume, and the
sugar is dissolved. The yolks should look light yellow.
Add the chocolate mix to the yolks, beating until blended. Then
add the flour, beating on low speed until thoroughly mixed.
Fold the mixture into the egg whites. When completely mixed,
pour into a very well greased 7 cup pudding basin, or a bowl
suitable for use on the stove. If using the bowl, construct
a lid as follows: Cut a piece of wax paper large enough to cover
the bowl, with some excess for folding. Fold a 1-inch pleat
in the middle of the wax paper. Do the same with a piece of
aluminum foil. Cover the bowl with the wax paper, then with
the foil, lining up the pleats. Secure with kitchen twine; cut
off any excess. You may wish to construct a lifter out of twine,
or you can use two spatulas to lift the pudding.
Pour about two inches of water into a pot large enough to fit
your pudding basin. Place the basin inside and cover the pot.
Keep the water in the pot boiling for about 1 1/2 to 2 hours.
(You will need to add more boiling water from the kettle from
time to time.) The pudding will be firm and cake-like on top
when it is finished. If you are not sure, boil it longer, as
it can't be overdone. Once it is ready, let it cool for
a few minutes in the basin, then turn it out on a plate.
Serve warm or let it "age" in an airtight container,
which will enhance the flavor.