• Preheat the oven to 180C / Fan 160C / Gas Mark 4. Line a 39cm x 27cm Swiss Roll tin with baking paper, leaving a good amount overlapping the edges. Set aside.

First make the sponge:

• Separate the eggs, putting the egg whites in one bowl and yolks in another. Using an electric whisk, whip the egg whites until they start to stiffen. Then sprinkle 50g of the caster sugar in, continuing to whisk while you do. Stop whisking when the mixture forms stiff, glossy peaks.
• In the other bowl, add the rest of the caster sugar to the egg yolks and whisk until they start to go pale and thick. Sift in the flour, cocoa and xantham gum, and fold in gently using a spatula. 
• Divide the egg white mixture into 3-4 and add a portion at a time to the egg yolk mixture, folding in gently. Be careful not to knock too much of the air out of it. 
• Once mixed, spread the mixture onto the lined baking tray, and bake in the middle of the oven for around 20 minutes, until the sides start to shrink away from the edge of the tray. Remove from the oven.
• Lay a piece of baking paper out on the worktop. Sprinkle a little icing sugar over the top and then turn the hot sponge straight out onto it. Remove the piece of baking paper from the bottom of the sponge.
• Score a line around 2-inches from the edge of the longest edge, and then carefully roll the sponge up with the baking paper inside, starting at this edge. Once completely rolled, rest with the rolled side down to keep it in shape and cool completely on a wire rack. 

Next make the frosting:

• Melt the dark chocolate (in a microwave or in a bowl over hot water) and then set aside to cool. Add the butter to a large bowl and beat with an electric mix until pale. Sieve in the icing sugar and beat again until smooth.
• Once the chocolate has cooled to room temperature, add it to the butter and icing sugar and beat until smooth and combined. 

To assemble:

• Carefully unroll the sponge and spread the inside with a thick layer of the frosting. Roll back up. If you want a branch like mine, cut off around 1/4 of the roll at an angle, and then place the angled end alongside the edge of the main 'trunk'. The end of the roll should be on the bottom.
• Using a spatula or knife, spread the frosting all over the log - don't forget the ends too! Then use a fork to create a bark-like effect. Sprinkle with a dusting of icing sugar and add your edible holly on top to finish!