Grease and line two 23cm / 9 inch cake tins with baking parchment or greaseproof paper.
Sift the flour and cocoa powder into a large bowl.
Add the caster sugar, baking powder and salt.
In a jug, measure the dairy-free milk, dairy-free yoghurt, oil and vanilla extract.
Pour the wet ingredients into the bowl with the dry ingredients, and mix well until the batter is smooth.
Divide the cake batter between the two cake tins, then give then a good firm bash down onto your work surface - this brings any of the air bubbles that are already forming due to the baking powder to come to the surface, which will give your cake a much more uniform texture.
Bake the cakes for 20-22 minutes, until an inserted knife or skewer comes out clean. Leave them to cool in the tins for a few minutes, then turn them out of the tin and when they are completely cool, peel the baking parchment off the bottom.
If at all possible, at this point it hugely improves the texture of your cakes if you store them overnight in an airtight tin or container. This brings a lot of the moisture from the cakes to the outer edge, and makes the sponge incredibly soft. If you don't have time to do this don't worry, your cake will still taste delicious, but do make sure it is absolutely 100% cool before spreading with icing.
To make the icing:
Sift the icing sugar into a food mixer or large bowl. Add the dairy-free margarine and vanilla extract and beat until soft and fluffy.
Melt the dark chocolate in the microwave (in short 20 second blasts, mixing between each blast), or in a bowl over a saucepan of boiling water. Set aside to cool for a few minutes. Meanwhile, get your cakes ready on the plate you will be icing them on.
Mix the chocolate into the icing, beating well.
Immediately spread one third of the icing over the bottom layer of the cake.
Place the second cake on top then spread the remaining icing over the top of the cake.
To make the chocolate decorations, use a potato peeler to scrape off little rolls from the remaining dark chocolate, and gently scatter over the cake.