Preheat the oven to 180°C/fan 160°C/gas mark 4. Lightly grease and line the bottom and sides of two 18cm loose-bottomed round cake tins and two 15cm loose-bottomed cake tins. Separate all the eggs. Mix 6 egg whites with the buttermilk and vanilla extract and set aside.
Add the butter and sugar to the bowl of a stand mixer and beat on a medium speed for 4-5 minutes until light and fluffy. Combine the flour, baking powder and bicarbonate of soda.
Put the mixer on a low speed. Add the dry ingredients in four additions, alternating with the buttermilk mixture, and beginning and ending with the dry ingredients.
Divide the batter evenly between the prepared cake tins. Bake for 25 minutes, until the smaller cakes are cooked. Remove from the oven and leave to cool on a cooling rack. Leave the larger cakes to cook for a further 5-7 minutes, then remove to the cooling racks. After 10 minutes, turn them out of their tins on to the cooling racks, letting the cakes cool completely.
Meanwhile, add 6 of the leftover egg yolks, the granulated sugar and the lime juice and zest to a medium pan. Place over a medium heat and whisk continuously for 5 minutes until thickened. Remove from the heat and add the butter 1-2 cubes at a time, whisking well between each addition, until thickened. Strain through a sieve so the curd is smooth. Pour into a bowl, cover with clingfilm so it touches the surface and transfer to your fridge.
Bring a medium pot filled with a few inches of water to a gentle simmer. Combine the 4 remaining egg whites and sugar in the bowl of an electric mixer, and place over the pot of water. Heat, stirring constantly, for 8-10 minutes until the mixture reaches 70°C.
When the egg and sugar mixture is the right temperature, transfer the bowl to the mixer fitted with the whisk attachment. Whisk on a high speed for 10 minutes until the mixture reaches stiff peaks and the bowl is no longer hot to the touch. The mixture should be smooth and glossy. Gradually add the butter in chunks. Continue adding and whisking until all of the butter is incorporated, then whisk in the gin and tonic flavouring. Continue whisking for a further 2-3 minutes until thick and smooth. Cover and set aside.
Divide the chocolate into different bowls, depending on how many different colours you want to create. Heat each bowl on a high heat in 10-second bursts, stirring in-between, until the chocolate has melted. Add a few drops of food coloring to each bowl and stir through.
Line two baking sheets with greaseproof paper. Using a 4cm wide, clean and unused paintbrush, paint strokes of the different coloured chocolate on to the baking sheet. Rinse out the brush until it’s completely clean and dry it off between each colour. Try and make all the brush strokes as lengthy as possible. Transfer the sheets to the fridge for 5 minutes.
Remove from the fridge and go over the brushstrokes with another layer of chocolate, being sure to wash and dry the brush when changing colour. Transfer to the fridge again for 5 minutes. Repeat the process twice more so that the brushstrokes have four layers of chocolate in total. Chill for 5 minutes, then brush a few of the white shards lightly with the gold paint brush. Transfer to the fridge until completely set and the cakes are ready to be decorated.
Bring the curd to room temperature and trim about 1½cm around the edge of each smaller cake. Level off the tops of each of the cakes to create a flat surface. Place one of the larger cakes on a cake board and add a layer of the frosting. Smooth it out so it’s even, then add a layer of the curd, making sure you leave a border of around 2cm. Add the other larger cake on top, pressing down lightly. Using some of the buttercream, crumb coat the cakes, making sure there's a thin layer of frosting all over the cake. Transfer to the fridge and leave to set for 30 minutes – this crumb coating will stop any stray crumbs from being visible in the final cake.
Repeat the buttercream and curd layering with the smaller cakes, then transfer to your fridge. Once they are both firm, remove from the fridge and cover with another layer of buttercream, using a cake scraper or off-set spatula and a cake turntable to create a smooth even layer. Transfer to the fridge and chill again before adding a final layer, smoothing it out completely.
Smear a dollop of buttercream in the centre of a cake stand. Using two fish slices, transfer the larger cake to the cake stand – the buttercream smear will help keep it fixed. Use the fish slices to very carefully transfer the smaller cake to the centre of the large one. If the buttercream smudges, add a little of any leftover frosting. Take the shards of chocolate and place them on the cake in your own design. Lightly dust the cake with the gold glitter spray to serve.