1. Preheat oven to 200°C/400°F/Gas 6. Line a swiss roll tin 33x23x3cm with greaseproof paper.
2. Put the egg whites into a large clean dry bowl, add a pinch of salt. Beat until mixture is stiff and dry looking in ap­pearance. Add the sugar and continue beating until thick and shiny. Finally beat in the egg yolks until thick and trail of beaters remain. Sieve the flour, cocoa and baking powder together and gradually add to the egg mixture folding it in with a metal spoon.
3. Transfer to the prepared tin and spread evenly. Bake on the top shelf for 10 minutes. Lay a sheet of greaseproof paper dusted with caster sugar on the work surface. When the cake is baked, turn it onto the paper, remove the lining paper, and trim the edges with a sharp knife. Roll the cake up from its longest edge with the paper inside. Leave to cool.
4. Whisk the cream until stiff. Remove the paper from the cake, spread the cream and carefully roll up again into a log.
5. Make the icing by beating butter, sugar and cocoa powder together until smooth. Once this is done spread the icing over the outside of the log.
6. Use a fork to mark the icing to give the effect of tree bark. Dust with icing sugar to resemble snow and decorate with holly leaves or fresh cranberries.

Note

• For larger log make 2 sponge rolls and use one as the tree and the other as the branch.