Preheat the oven to 180 degrees or gas mark 4. Grease and line a swiss roll tin with baking paper and dairy free margarine.
Using an electric hand whisk, beat together the eggs and sugar until light and fluffy.
Sift the gluten free self-raising flour, cocoa powder and xanthan gum and carefully fold into the mixture with a large metal spoon.
Pour the mixture into the swiss roll tin. Spread into each corner.
Bake for 10 minutes.
Remove the cake from the oven and turn out (upside down) on a piece of baking paper, peel the original paper off.
Score a line, about 2cm in, on the longest edge of the sponge. Then carefully roll the sponge, with the paper attached, to form the swiss roll. Leave to cool.
Make the icing by mixing together 300g icing sugar and margarine. Once you have the right consistency, add the cocoa powder. This will be used on the inside of the log.
Carefully unroll the sponge, remove the baking paper and spread an even layer of icing.
Roll up the cake and place it on a board to serve. Melt 100g dark chocolate and add this to the icing you've already prepared. Add 100g icing sugar to thicken the icing.
Cut a third of the cake off at an angle, this will form the additional log, and place the angled edge alongside the main log.
Spread icing all over the log until completely covered, then score the bark effect using a skewer or cake tester.
Dust with icing sugar, add a decorative holly made from fondant icing then serve.