Separate the eggs making sure that the whites have absolutely no yolk in them. Put the whites in one stand mixer bowl (use the smaller one if they are different sizes) and the yolks in the larger bowl. Set aside.

In a saucepan set over low heat, melt the chocolate with ¼ cup water, stirring until smooth. Let cool for about 10 minutes.

Set the stand mixer bowl with the egg whites on the mixer stand and with the whisk attachment, beat on medium until they are frothy. Add the cream of tartar, and beat until soft peaks form. Add ⅓ cup of the sugar gradually and beat just until peaks start to hold stiff peaks -- don't overbeat.

Remove the bowl of egg whites from the stand mixer and replace with bowl with the egg yolks. Using the same whisk attachment (no need to clean it), beat the egg yolks with remaining ⅓ cup of the sugar for 5 minutes, or until the mixture is thick and pale and ribbons form when the beaters are lifted. Beat in the salt, vanilla and melted chocolate.

With a heavy scraper, fold the egg whites into the egg yolk mixture in 3 parts.

Pour the batter into the pan and spread it evenly. Bake on the center rack for 12 to 15 minutes, or until it is puffed and just set. Let the cake cool completely in the pan on a rack. It will sink slightly as it cools.

Filling: When cake is cool, prepare the filling. In a chilled mixing bowl combine the cream, the espresso powder, and the cinnamon and beat the mixture until it holds soft peaks. Add the confectioners' sugar and beat until mixture holds stiff peaks.

Cake Dusting Mixture: In a small bowl stir together the cocoa powder and the confectioners' sugar and sift about half of the mixture evenly over the cooled cake. Cover the cake with a sheet of wax paper, then invert onto a baking sheet so that it sits on the wax paper. Peel off the foil and the original wax paper carefully and mound the filling lengthwise down the center, leaving a 1-inch border at each end. Starting at the short side, roll up the cake jelly-roll style, slightly lifting the wax paper to help guide the cake into a roll. It will crack slightly, but that is okay. Bring the waxed paper up and around the roll and chill the cake for at least 1 hour and up to 2 hours. At this point, I think you could freeze it as well. I will test that at some point.

Transfer the cake to a platter, remove the wax paper, and trim the ends on the diagonal. Sift the remaining cake dust mixture over the top.

Make the ganache. Put the cream in a small cup and microwave for 30 seconds. Add chocolate and stir until smooth. Let it cool down a bit -- it will thicken as it cools. Put it in a decorating bag or freezer bag, snip off the tip and squeeze decoratively over log. it will slip off the sugar, but if you let it thicken you should be able to coax it into sticking. Decorate with sweetened whip cream if desired.