Line a 40x25cm baking tray with baking paper; grease the paper. Place egg whites in a small mixing bowl; let stand at room temperature for 30 minutes. In a large mixing bowl, beat egg yolks on high until light and fluffy. Gradually add 100g sugar, beating until thick and lemon-colored. Combine flour, cocoa and salt; gradually add to egg yolk mixture until blended.
Beat egg whites on medium until foamy. Add cream of tartar; beat until soft peaks form. Gradually add remaining sugar, beating on high until stiff peaks form. Stir a quarter into chocolate mixture. Fold in remaining egg whites until no streaks remain.
Spread batter evenly in prepared pan. Bake at 180 degrees C for 12-15 minutes or until cake springs back (do not overbake). Cool for 5 minutes; invert onto a linen tea towel dusted with icing sugar. Peel off baking paper. Roll up in the tea towel, starting with a short side. Cool on a wire rack. In a mixing bowl, beat cream until it begins to thicken. Add sugar and coffee granules. Beat until stiff peaks form; chill. Unroll cooled cake; spread filling to within 1 cm of edges. Roll up again, without the tea towel. Place on serving platter; chill.
In a mixing bowl, beat icing ingredients until smooth. Ice cake. Using a fork, make lines resembling tree bark.