Position rack in the middle of your oven. Preheat 350°F.
In a double boiler melt the bittersweet chocolate with the warm water. Let cool at room temperature.
Grease the bottom of a 18 x 13 inch rimmed baking sheet with the softened butter. Line the pan with parchment paper, then with the rice flour lightly flour the parchment paper.
Separate your 9 eggs from the yolks and the whites. With an electric mixer whip the egg yolks in a large bowl about 3 minutes until light and pale yellow. Mix in the melted bittersweet chocolate. Then stir in the 1 ounce of cocoa and the salt.
In a clean glass bowl whip the egg whites for two minutes until frothy. Slowly add the white sugar, streaming it into the egg whites while you are beating them. You want a soft peak so make sure you beat for 3 more minutes if using an electric mixer.
With a rubber spatula fold the eggs whites into the chocolate mixture slowly. Do not over mix.
Scrape the mixture into the baking sheet. Gently spread the mixture and smooth it to make sure it is level. Bake for 22 minutes.
Meanwhile spread a clean dishtowel (at least as big as the cake pan) dust the towel with the 1/8 tsp of cocoa powder. This will keep the cake from sticking to the towel.
Immediately after taking the cake out of the oven, run a knife around the edge of the pan to loosen the cake from the edge. Invert the cake onto the towel in a quick motion. Carefully peel off the parchment paper. Using both hands and starting from one end to the other. Then while the cake is still hot gently roll up the cake and the towel together. Let cool at room temperature.