Preheat oven to 180°C and line a Swiss roll tin with baking paper.
Separate the eggs, adding the egg whites to 1 bowl and egg yolks to another bowl. Whisk the egg whites with an electric whisk until you reach stiff peaks (this will take a few minutes). Add 50g of the caster sugar to the egg whites and continue to whisk until glossy. Set aside.
Add the remaining 75g of caster sugar to the egg yolks and whisk with an electric whisk until pale and frothy. Sift the gluten free plain flour, xanthan gum and cocoa powder into the egg yolk & caster sugar mixture. Fold in using a spatula.
Fold the egg whites a little at a time into the egg yolk mixture until fully incorporated (try to be gentle with this so you don't loose too much air from the egg whites).
Pour the mixture into your tin, spreading it out to the edges. Bake for 10-12 minutes until firm but still springy.
Remove from the oven and whilst it’s still warm, turn out onto another large sheet of baking paper sprinkled with a little icing sugar. Carefully peel the piece of baking paper off the bottom of the cake and trim the edges of the cake so they are straight.
Whilst the cake is still hot, tightly roll the sponge up from the short end with the baking paper inside it. Leave to cool completely.
Whilst the cake cools make the buttercream, by beating dairy free butter with an electric whisk until creamy. Slowy add the icing sugar and continue to whisk for up to 5 minutes until creamy and combined . Add the melted dark chocolate and milk and whisk again until combined.
Carefully unroll the sponge and spread with a layer of the buttercream. Roll back up and cover with the remaining buttercream. Use a fork to create a log pattern and dust with icing sugar to serve.