Preheat oven to 180ºC (350ºF). Grease and line a 15 X 10-in. (38 X 25-cm) baking sheet with parchment paper, with overhang on 2 sides only. Beat egg yolks and 1/2 cup (125 mL) sugar with electric mixer until pale yellow and tripled in volume. Fold in the ½ cup (125 mL) cocoa powder.
In another bowl, with electric mixer (and clean beaters) on high speed, beat egg whites and salt until soft peaks start to form. Gradually add remaining sugar, beating to stiff peaks. Fold one-quarter of egg white mixture into yolk mixture. Then gently fold in remaining egg white mixture; do not overmix or egg whites will deflate.
Spread batter evenly on lined baking sheet. Bake 12 to 15 min. until puffed, top springs back when lightly touched and cake-tester inserted into centre comes out clean. Remove from oven; cool completely.
Lightly dust cake with cocoa powder. Run a knife around edges to loosen from parchment and baking sheet. Carefully invert onto a clean sheet of parchment paper. Remove the parchment from top of cake.
Using electric mixer (clean beaters), beat whipping cream to stiff peaks. Sift in icing sugar and beat just to blend in.
Spread half the whipped cream mixture evenly over cake, leaving a 1-in. (2.5-cm) border around the edge. Beginning at a long side, use the parchment to lift and tightly roll up cake. Transfer the cake log to a serving platter, seam-side down.
Reserving about 2/3 cup (150 mL) whipped cream mixture, spread the rest over the sides and ends of the log. Lightly dust top of log with cocoa powder. Pipe or spoon the reserved whipped cream mixture in 8 staggered dollops along the top of the log. Break bark into 8 shards; lean them on the whipped cream dollops. Also decorate with a few blackberries. Chill until serving time.
The log can also be made without the whipped cream icing on top. Just make enough for the filling by reducing to 1 ½ cups (375 mL) whipping cream and ¼ cup (60 mL) icing sugar. Roll the log, dust with icing sugar and garnish with a few berries.