Wrap both cakes in clingfilm and place in the fridge for 30 minutes.
Icing
Place the butter in a bowl and beat until light and fluffy, about a minute.
Add the icing sugar and beat until thoroughly combined and smooth. You'll end up with a very pale cream mix.
Add the ginger syrup and lemon curd and beat on the lowest setting to mix.
Keep it covered with cling film until you're ready to use it.
Decorating the Cake
At the end of the 30 minutes, level the tops of both cakes, they want to be flat, with any crumbly bits removed.
Place one levelled cake on a cake stand and spread the icing all over.
Place the second cake on the icing.
Now check that your cakes are sitting perfectly in line with each other, with no parts sticking out. If need be, trim with a serrated knife for a completely smooth alignment, as mentioned above.
Take the remainder of the icing and heat it in the microwave at 50% power for 6 seconds. Stir thoroughly (We heat the icing slightly to allow for easier spreading).
Now, take your spatula and spread the icing all around the sides of the cake.
When the sides are done, do the top.
When done, hold your scraper flush against the side of the cake with your dominant hand, and run it along the cake edge, as you turn your cake stand, scraping away as much of the icing as you can. Without taking away any cake! A revolving cake stand is handy but not necessary. You can see that I'm not using mine, just a regular cake stand. Clean the icing off the scraper and repeat a couple more times until you are happy that you can't get anymore icing off.
Now, level the top of the cake with the scraper, but don't worry about a smooth surface, that's no longer the fashion!
Then decorate as you like. Break some of the chocolate pieces and biscuits up into varying lengths and insert into the cake as I've done. Finish with some of the ginger pieces and grate some dark chocolate all over, if you like. I used one of the chocolate biscuits for that!
Notes
The time here is only for assembling the cake. We are not taking into account the ginger cake and the icing.