A chocolate roll cake filled with vanilla buttercream and coated with ganache.
Preheat your oven to 180°C (or 160°C if using a fan or convection oven). Line a 10 x 14-inch tin with baking paper, leaving a small amount hanging over the edges to make for easy removal.
In a large bowl, mix together the flour, cocoa powder, baking soda and xanthan gum.
Stir in the caster sugar.
Add the melted margarine, soy milk and vanilla, then mix well until you have a smooth batter.
Transfer the batter to your lined tin and spread it out to the edges. It will be thicker than a regular cake mix so try to make sure it's spread very evenly.
Tap the tin on the counter a few times to get rid of any large air bubbles, then bake for 12 minutes.
Dust a clean dishcloth/tea towel with unsweetened cocoa powder. You'll need a decent amount but don't go too crazy or the cake will taste bitter. Do not use baking paper in place of a dishcloth as it will not work.
Let the cake sit at room temperature for 5 minutes so that it's cool enough to handle. Then carefully tip it onto the dishcloth.
Sprinkle a light dusting of cocoa powder onto the cake. Cut off any rough or uneven edges using a knife.
Carefully use the dishcloth to roll it up, with the dishcloth in between the rolled cake. See pictures in the attached post if you're unsure.
Let the rolled-up cake sit at room temperature for 90 minutes to cool.
Gently mix the margarine in a bowl to soften it. Add in the icing sugar a little bit at a time until smooth, using the soy milk to loosen it if needed.
Add the vanilla extract and mix it in well.
Whisk for a minute or so with an electric mixer until the buttercream becomes pale and fluffy.
Once the 90 minutes are up, very carefully unroll the cake. There shouldn't be any cracks but don't worry if there are a few small ones as they can be covered.
Spread an even layer of buttercream over the unrolled cake. Gently roll it back up, this time without the dishcloth.
Use a pastry brush to dust any excess cocoa off of the cake then cut a slice off of each edge to make it look neat.
Cover it in plastic wrap and transfer it to the fridge for 1 hour, which will help it to hold its shape.
Chop the dairy-free chocolate VERY finely and add it to a glass bowl with the soy milk.
Place the bowl over a small saucepan of water, so that it sits on top of it without touching the water.
Put the saucepan and bowl over a low heat and gently stir until the chocolate has completely melted and there are no lumps left.
Use a dishcloth or oven gloves to remove the bowl from the saucepan and let it sit at room temperature for exactly 1 hour.
After the hour is up, use an electric whisk to whisk it for around 10 seconds. This will make it light and fluffy. Be careful not to mix for too long or the ganache will be too firm to pipe.
Transfer the ganache into a piping bag with a large star nozzle. Pipe it onto the yule log in vertical lines until it's completely covered.
Add a light dusting of icing sugar to finish if desired. I also added some artificial berries and greenery for presentation.