Preheat your oven to 170ºC/150ºC Fan, and line two 8"/20cm cake tins with baking parchment.
In a stand mixer, or a large bowl, beat together the unsalted butter and light brown soft sugar until light and fluffy.
Add in the eggs, self raising flour and vanilla extract and beat again until combined well.
Split the mixture into two bowls - they will weigh roughly 1000g each. Add the chocolate chips to one and stir, and add the cocoa powder to the other and stir.
Add each mixture to a prepared tin.
Bake the cakes in the oven for 50-60 minutes, or until baked through - check with a skewer to make sure they're done!
Leave the cakes to cool in the tins for 30 minutes, and then take out and leave to cool fully on a wire rack!
For the Buttercream
Beat the unsalted butter on its own for minute or two, to soften it and loosen it.
Add in the icing sugar and vanilla extract and beat well until smooth and fluffy. I also coloured my buttercream golden using 1/2tsp of amber food colouring but this is optional.
For the Decoration
Once cooled, split each cake into two layers each.
Put the first cake onto a cake board of plate. Add a little buttercream and spread. Repeat with the second and third sponges. Add the final sponge on top. I did chocolate first, then chocolate chip and repeated.
Using a small amount of buttercream, spread and smooth around the cake sides and top for a crumb coat and add to the fridge for at least 30 minutes.
Once set, add more buttercream onto the tops and sides of the cake, and smooth around covering the cake completely in buttercream.
I add the buttercream on using a small angled spatula, and smoothed around the edge with the large metal scraper! You need to make sure there is more buttercream on the sides of the cake than you need, as some will be removed as you smooth it over.
Once finished, press the cookie crumbs into the base of the cake using the palm of your hand - this process is a little messy, but worth it!
Put the cake in the fridge again for at least 30 minutes.
Ganache
Add the dark chocolate, milk chocolate and double cream to a bowl, and microwave until smooth.
Using a small piping bag or a teaspoon, drip the ganache down the sides of the cake. The more you push over the edge, the further the drip will fall.
Set the cake in the fridge again for 30 minutes!
Decoration
Finish the cake off by piping on any leftover buttercream, and adding some whole cookies, and half cookies like I have, and then also some extra cookie crumbs!