Preheat oven to 200°C/400°F/Gas 6. Line a swiss roll tin 28cm x 18cm/11” x 7” with greaseproof paper.
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 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.
Transfer to the prepared tin and spread evenly. Bake 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.
Make the icing: Beat margarine, sugar and cocoa together until smooth.
Whisk the cream until stiff. Remove the paper from the cake, spread the cream and carefully roll up again into a log.
Cut a thick diagonal slice from one end of the log. Lift the log onto a plate, then arrange the slice on the side with the diagonal cut against the cake to make a “branch”.
Spread the icing over the “log” and the “branch” (don’t cover the ends).
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.
Note: for larger ‘log’ make 2 sponge rolls and use one as tree and the other as the branch.
Put the cream, the caster sugar and the split vanilla pod in a saucepan and slowly bring up to a simmer. Turn off the heat and leave to infuse.
Meanwhile, put the gelatine leaf into a bowl with just enough water to cover it. After 4-5 minutes it will have softened completely. At that point, take the softened gelatine out of the water and put it into the saucepan with the hot cream mixture. Remove the vanilla pod. Whisk gently until the gelatine dissolves. If the cream has cooled down too much, you might need to reheat it slightly with the gelatine in it. Pour the panna cotta into ramekins or glasses, then place them in the fridge for about two hours until the panna cotta is set. Serve with fresh raspberries and mint leaves.