Make up your chocolate buttercream by combing all the ingredients and whisking with an electric whisk.
Using half of the total buttercream, pipe onto each layer of cake to make the filling, stacking the next cake on top.
Using a quarter of the remaining buttercream ,spread over the top of the cake and down the sides. Do this sparingly as it’s only the crumb coat it should be no more than 1cm thick.
Place into the fridge for 15-30 minutes to harden.
Once the buttercream is set remove from the fridge and apply the final layer of buttercream make this as neat as possible. I find a plastic cake scraper is the best for this.
Place back into the fridge for a further 30 minutes.