1. Heat oven to 375 degrees F. Line a 15x10x1-inch jelly-roll pan with wax paper; grease paper. Sprinkle paper lightly with flour, shaking out excess.
2. Sift 1/2 cup of the cocoa, the 1/3 cup flour, the baking soda and salt into a bowl.
3. Beat egg yolks and 1/3 cup of the sugar in large bowl at medium-high speed of electric mixer 5 minutes or until thick and pale gold. Set aside.
4. Beat egg whites in another large bowl at medium-high speed until foamy. Gradually add remaining 1/2 cup sugar; beat just to stiff peaks. With rubber spatula, fold one third of whites into yolk mixture to lighten. Fold in remaining whites in two additions, just until blended. Sift one third of cocoa mixture over yolk mixture; gently fold in until almost blended. Repeat folding in remaining cocoa mixture in two additions just until blended. Gently fold in grated chocolate. Spread batter evenly in prepared pan. Bake 11 to 12 minutes or until center springs back when lightly pressed with fingertip.
5. Meanwhile, sift remaining 3 tablespoons cocoa over a clean kitchen towel to cover surface. When cake is done, immediately invert onto towel, carefully remove pan and peel off wax paper. Lifting from the long side, roll hot cake up with towel jelly-roll fashion. Cool cake in towel completely.
6. Refrigerate ice cream 20 to 25 minutes to soften.
7. Unroll cooled cake. Gently spread softened ice cream over top, then re-roll cake (dont worry if it cracks slightly). Transfer cake, seam side down, to serving plate. Cover with plastic wrap and freeze 4 hours or until firm.