Preheat your oven to 180C/160C Fan, and line three 8"/20cm cake tins with baking parchment.
Melt your dark, milk and white chocolates carefully, and leave to cool whilst doing the cake mix!
In a stand mixer, or a large bowl, beat together your butter and sugar until light and fluffy and lovely!
Add in your eggs, baking powder, and flour, and milk and beat again until combined well.
Split the mixture between three bowls, and to each bowl add one of the melted chocolates - mix until smooth.
Add your dark chocolate mix to one tin, your milk chocolate to another tin, and the white chocolate to the last tin.
Bake the cakes in the oven for 35 minutes, or until baked through - check with a skewer to make sure they're done!
Leave the cakes to cool in the tins for 10 minutes, and then take out and leave to cool fully on a wire rack!
For the Buttercream
Melt your dark, milk and white chocolates carefully, and leave to cool whilst doing the buttercream!
Beat your butter on its own for minute or two, to soften it and loosen it.
Add in your icing sugar 1/3 at a time, and beat fully after each addition - you shouldn't need to add any liquid in!
Take out about 150g of buttercream and leave separate. (This is for a crumb coat)
Split the rest of the mixture evenly between three bowls, and just like the cake mix, add each chocolate to its own batch of buttercream.
Mix fully, until delicious.
Assemble
I started with my dark chocolate sponge on the bottom, and spread about 2tbsp of buttercream onto the cake.
My milk chocolate cake went second, and the white on top!
Use the spare buttercream to make a crumb coat around the cake and to make it smooth!
Add each flavour of buttercream to a large piping bag, and pipe rings around the cake - I did two layers of buttercream for each layer of cake (this is so hard to describe)
Once the three flavours are on the cake, scrape around the cake just like you would other drip cakes, and smooth it all over! Be careful when you are doing it as you'll be able to save the rest of the buttercream for piping!
Ganache
Add your chocolates and cream to a microwave proof jug/bowl, and heat for 30 seconds. Stir well, and microwave on 10 second bursts till the ganache is smooth, stiring each time.
Add your ganache to a piping bag and snip a small amount off the end of the piping bag
Pipe drips down the side of the cake, and then fill in the top and spread it over.
Decoration
Leave this to set for a minute, and then pipe the rest of the buttercream on top as decoration!
I finally sprinkled on some triple chocolate curls!
Notes
I use supermarket own chocolate for all three layers, I wouldn't recommend stuff like Galaxy or Dairy Milk for the milk layers - just go with the cheap!
You can just slather the buttercream on in any which way, but I have tried to describe it as best as possible. You basically want the buttercream of each flavour, to match each sponge, and then wipe round so its all smooth!