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1. Preheat the oven at 200^oC (400^oF). Butter a jelly roll pan. Line it with greased parchment paper.
2. Beat the egg whites using an electric beater till firm.*
3. Beat the egg yolks with the electric beater along with the powdered sugar and 3 tablespoons of warm water to lather. Add gradually, the flour and the baking powder, mixing well. Fold in very gently  the beaten egg whites.
4. Bake for about  10 minutes only. Take care to not overbake or else the cake may become too stiff to roll without breaking. 
5. Turn the cake out onto a clean wet dishtowel. Roll up and cool slightly..
6. Place the grated chocolate in a steel bowl. Hold the bowl over a pan of simmering water (double boiler or bain-marie) and stir till it melts and smoothens. Add the melted butter and mix well. Spread the chocolate-butter mixture over entire surface, and roll the cake gently. Place the rolled cake on a serving plate.
   
7. To frost the outside of the log, spread the rest of the butter-chocolate mixture on top and, using a fork, trace irregular lines in the frosting to give it a woody effect. Refrigerate the cake for 20 minutes to set the frosting, then cover loosely with plastic wrap and allow it to 'mature' in the refrigerator for several hours or overnight. 

• * To separate the eggs effortlessly, use refrigerated eggs. The yolk is less likely to break up when the eggs are cold. However, once the cold eggs have been separated, they must be used only when they come up to room temperature.
• * When beating egg whites, it is advisable to begin at a slow speed and gradually move up to high, taking care to stop once they are firm. Use the beaten egg whites right away. Do not let them sit.
• For baking cakes, it is recommended that all the ingredients used are at room temperature unless stated otherwise.