Preheat the oven to 180C and line 2 round 20cm cake tins.
To a large bowl add all your dry ingredients (flour, xanthan gum, baking powder, bicarbonate of soda, salt, cocoa powder, golden caster sugar and light brown sugar). Mix well until everything is combined.
To the bowl add the eggs, soy milk and vegetable oil. Whisk well until everything is completely incorporated and you have a nice chocolatey mixture.
Slowly add the hot coffee, whisking as you go until it's combined in the mixture. The mixture will be quite runny - this is what you want to make the perfect chocolate cake.
Pour the mixture into the 2 prepared cake tins and bake in the middle of the oven for around 35 minutes, or until an inserted skewer comes out clean from both cakes (this may be slightly longer or shorter depending on your oven so keep an eye on it).
Remove from the oven, leave In the tins for 15 minutes and then transfer to a wire rack to cool completely.
Once cool make the buttercream. Add the dairy free butter to a large bowl and beat until creamy. Gradually add the icing sugar, continuing to beat until fully combined and creamy. Add the cocoa powder and splash of milk and beat again until combined.
Transfer the buttercream into a piping bag (optional - you can spread the buttercream onto the cake but piping is neater) and pipe onto one of the cakes, place the other cake on top and then pipe the rest of the buttercream onto the top.
Store in an airtight container and consume within 5 days.