When your cakes are completely cold, use a carving knife to level them. Just trim the top crust off. This is important, it’s not a waste of cake (taste testing compulsory). If you don’t do this, with a cake this size it will be wonky.
Drizzle any remaining syrup over the cakes (there won't be loads, but a few drops here and there is just enough.)
Sandwich each layer together with some of your buttercream (I alternated colours) and a layer white buttercream on top.
Next, using a palette knife, cover the outside of the cake, up to a third of the way up the cake with the deepest pink icing. (Reserve a little of this icing.)
Then, cover another third up the cake with a good layer of the lighter pink icing.
Finally, finish the final third of the outside edge of the cake with the white icing.
Now run a clean, smooth scraper or palette knife around the cake, smoothing and slightly blending the 3 icings.
Pipe 16 spots of the remaining coloured icing around the top of the cake and pop a blackberry in each one.