Preheat the oven to 160c fan and grease and line 4 round cake tins.
For the vanilla and dark chocolate cakes: beat together the butter and sugar. Whisk together the buttermilk and eggs. Mix together the flours (all in separate bowls). Once the butter / sugar is creamed, add the flour and the egg mixture in alternating portions until all combined. Split the mixture in two; add vanilla extract to one half and cocoa powder to the other. Pour into tins and smooth.
For the caramel and milk chocolate cakes: beat together the butter and sugars. Whisk together the buttermilk and eggs. Mix together the flours (all in separate bowls). Once the butter / sugar is creamed, add the flour and the egg mixture in alternating portions until all combined. Split the mixture in two; add vanilla extract to one hafl and cocoa powder to the other. Pour into tins and smooth.
Bake the cakes for 30 minutes or until springy and golden. Cool completely and trim if necessary.
To make the dark chocolate ganache, heat the chocolate, cream, milk and syrup in a bain marie. Remove from the heat and stir in the butter. Cool until spreadable.
To make the caramel ganache, heat the white chocolate with the caramel as above. Cool.
Do the same for the milk chocolate ganache.
N.B While assembling the cake, line up the layers so they are as flat as possible; trim if necessary. It can help to chill for 10 minutes once the caramel layer is in place as this will help it set and take the weight. If at any point the cake becomes unwieldy, chill.
Place the dark chocolate cake on a plate and spread over the caramel ganache.
Next add the milk chocolate cake and spread with milk chocolate ganache.
Add the caramel cake and a layer of dark chocolate ganache.
Top with the vanilla cake and then cover the whole cake with the dark chocolate ganache.
Decorate with chocolate buttons / scrolls / salted caramel pieces and chill overnight to set before trying to cut. Take out of the fridge about an hour before you want to serve it.