For the cake
Preheat the oven to 180°C/350°F/gas mark 4. Grease and line the base of two 20 cm cake tins.
Sift the flour, cocoa, bicarbonate, and salt together, then stir in the sugar.
In a separate bowl, combine milk, melted dairy-free margarine, yoghurt, and vanilla.
Make a well in the centre of the dry ingredients, pour in the wet ingredients and gradually combine until you have a lovely smooth batter.
Pour into the cake tins and bake for 25-30 minutes, until a knife comes out clean.
Cool in the tins for a few minutes, then turn out onto a wire rack.
For the buttercream
Whisk the dairy-free butter until light and fluffy.
Add the icing sugar 100 g at a time with a splash of the milk and vanilla, mixing until fully combined and fluffy.
If you want to use fondant icing for the decorations, use 500 g to cover the cake and any offcuts for the decorations.
Variations: Turn this into a chocolate fudge cake by replacing 25g of the icing sugar in the buttercream with cocoa powder.