Heat oven to 325 deg. F with rack in lower middle position. Line bottom of 8 inch springform pan with parchment paper, (wax paper will do, bit it sticks to the finished cake somewhat.) Grease sides of pan. Wrap underneath and sides of pan in aluminum foil and set in roasting pan large enough to allow a boiling water bath to surround the cake pan.
Beat eggs at high speed with electric mixer (speed 6 on a Kitchen-Aid) until volume doubles to about 1 quart - this takes about 5 minutes. Do not stint on this step as this air beaten into the eggs is what allows the cake to rise.
Melt chocolate and butter in the microwave (4-6 minutes) or over a double boiler until smooth and warm, but not boiling. Use coffee or liqueur to aid melting of desired, but it's not really necessary.
With a rubber spatula, gently fold 1/3 of egg foam into chocolate/butter mixture until a few streaks of egg are visible. Fold in half of remaining foam, then last of foam, mixing gently until all trace of egg is gone and batter is homogeneously chocolate colored.
With the spatula scrape batter into prepared springform pan and smooth top. Place pan in roasting pan, place in preheated oven and pour boiling water into roasting pan until it comes about halfway up the cake pan's side.
Bake until cake has risen slightly and edges are just beginning to set. If you have an instant read thermometer, insert it into the center of the cake (not touching the bottom of the pan.) When the cake is sufficiently cooked, the thermometer should read 140 deg. F. The cake will look completely uncooked and will be jiggly in the middle, almost as it went into the oven. In an 8 inch pan, cooking time is 22 to 25 minutes. An 8 & 1/2 or 9 inch pan will take 18-20 minutes. If you have no instant-read thermometer, go strictly by the time.
Remove the cake pan from the water bath and allow it to cool to room temperature on a wire rack. When cooled, it will still be sort of jiggly, but firmer than when it came from the oven. Cover and refrigerate overnight to firm up and mellow.
Before serving, cover cake top with a sheet of wax or parchment paper and a plate larger than the cake pan. Invert, loosen spring on pan and remove sided section and then bottom section of pan. If you used wax-paper as the separator between the cake and pan bottom, the wax paper will probably not come off in one piece. Peel it carefully off, place serving plate on the bottom of the cake, invert and sprinkle top with powdered sugar or cocoa if desired.
Serves 16-18 thin slices. It is incredibly rich, has a thick mousse-like texture, and a little is all most people can eat at once. It will keep several days in the refrigerator, if it doesn't disappear to people "straightening the edges."