Beat the unsalted butter on its own for a while in your mixer until its smooth and supple and starting to whiten.
Add in your icing sugar a little at a time and beat fully until its light and fluffy and add the vanilla extract and white food colouring!
(If the mixture is really stiff, add in 1-2tbsp of boiling water and beat fully to smooth out)
Add a few spoonfuls of the buttercream to a new bowl and colour this to a sage green.
Decoration
Level the cakes off if necessary.
Add your first cake to a cake board or flat serving plate and secure with a small amount of buttercream.
Spread some buttercream onto the first layer, add the second sponge. Repeat until you reach the top layer of cake.
Spread some buttercream around the sides to fill in the gaps and create a crumb coat. Refrigerate the cake for at least 30 minutes.
Once refrigerated, using an angled spatula, spread the rest of the white buttercream around the cake and then smooth around using a large metal scraper.
Chill the cake again for another 30 minutes.
Once chilled, carefully dab on the coloured buttercream, and then smooth around using a large metal scraper. You really do not need much coloured buttercream to get the effect.
Refrigerate the cake for another 30 minutes.
In a small bowl or jug, add the milk chocolate, dark chocolate and double cream and melt carefully in short bursts in the microwave, stirring well each time, until smooth.
Carefully pipe this onto the cake to create a drip - I chose random sections to make the look of the cake. Set the cake in the fridge for another 30 minutes.
Pipe some swirls of the leftover buttercream on top using a Medium 2D Closed Star Piping Tip and decorate with the sprinkles, macarons and decorations.
Notes
This cake will last for 3-4 days once made.
You can look at this post to see the details on making an 8" cake - or a chocolate cake version.