Into a medium bowl or tall jug, add the chopped chocolate - be sure to use 70% cocoa solids.

Then, add the milk, cream, sugar and egg yolks into a medium saucepan. Place this on a medium/medium-low heat.

Whisk the mixture constantly until it reaches 80-82C (176-179F) on a digital thermometer. Immediately remove it from the heat and pour it through a sieve, over the chopped chocolate. Don't worry if a little bit of egg catches on the bottom of the pan. 

Let that sit for a few minutes then using a hand blender, blend it until it is smooth. Stir through a little pinch of flakey sea salt. Pour it into a baking tray or dish and cover the surface with clingfilm. Allow it to chill in the fridge for about an hour, or until it is firmer to the touch but has a pipeable consistency. If it goes too firm, leave it out at room temperature until it softens. 

Add the chocolate into a bowl, and melt this over a pan of gently simmering water. Once melted set it to one side. We need this to cool slightly but still be runny when we pour it into the buttercream. 

Into the bowl of a stand mixer, add the egg yolks and whisk on a medium speed. 

Meanwhile, add the water and then sugar into a small saucepan. It's important to use a small saucepan here- it's a pretty small quantity of sugar syrup so it will be difficult to read the temperature accurately if you spread it too thin in a big pan. 

Heat the sugar syrup until it reaches 114C/237F on a digital thermometer. Immediately remove it from the heat and slowly drizzle it over the whisking egg yolks - which at this point, should be much thicker and fluffier in consistency. Try to avoid splattering the sugar syrup all over the whisk. You want to pour it down the side of the bowl. 

Once you've added all the sugar syrup, continue to whisk on a medium speed for around 5-6 minutes, until the mixture is much thicker and paler in consistency, and more importantly, the bowl feels cool to the touch.

Next, add in the soft butter, a piece at a time whisking for a few seconds in between each addition. 

When you've added all the butter continue to whisk for about a minute, then switch to the paddle attachment, and while mixing on a medium-low speed, drizzle in the melted chocolate followed by a pinch of sea salt flakes. It should result in a really smooth buttercream. 

Remove the chilled cake layers from the fridge and add the cremeux and buttercream to separate piping bags fitted with large round tip piping nozzles. 

Glue the first layer of cake down to a cake board with a little bit of buttercream, then pipe alternate swirls of the cremeux, then buttercream. Starting in the centre and spiralling your way to the edge of the cake. Stack the second layer on top and repeat the swirls of piping.

With the third layer of cake, flip it over so the smooth side that was touching the tin is facing up and place this on the top of the cake - this will give you a nice flat top (I forgot to do this!!). Refrigerate the cake while you make the ganache. 

Add the chocolate into a bowl and place this over a pan of gently simmering water. Stir it until it has completely melted and then set it to one side. 

In a saucepan, add the cream, butter, sugar and glucose (or honey). Stir this over a medium heat just until the butter has melted and the mixture is steaming. 

While constantly stirring in small circles in the centre of the melted chocolate, slowly pour in the hot cream until you've added it all. At this point, the ganache will most likely look split and grainy - that's fine!

Add in the sour cream and use a hand blender to emulsify the ganache, this will make it nice and smooth. Finally, fold in a pinch of flakey salt. Pour this into a baking tray and cover the surface with cling film. Leave it at room temperature and don't touch it, it will take about 2-3 hours to firm up slightly (on a 'normal' day - if it is cold it will be quicker). You are looking for it to reach the consistency of buttercream. If you try and refrigerate it and speed this up, you risk splitting the mixture so it is best left at room temperature.

Once the ganache has firmed up into a spreadable consistency, remove the cake from the fridge and apply an even coating of the ganache around the outside of the cake for a rough crumb coat. Use a palette knife and a bench scraper to get this as even as possible. Refrigerate the cake for another 15 minutes. 

Remove the chilled cake, and apply a final layer of ganache to the outside edge of the cake (don't worry about the top as we will cover this with the drip). It can be relatively rough so don't worry about it being neat.

Gently heat a metal combed cake scraper and run this around the outside edge of the cake a few times. I try not to do too many attempts in case I scrape off too much ganache (!) but 2-3 goes should be enough to get a nice decorative edge. As long as you have a nice front to the cake, don't worry about a few ugly patches at the back! Place the cake back in the fridge while you make the drip. 

Add the chocolate and butter into a bowl. Place this over a pan of gently simmering water and stir it until it as completely melted. 

Remove it from the heat and allow it to cool. You want it to be around 35-40C - the hotter the mixture, the more runny it will be so it will drip more. A cooler mixture will result in a shorter drip. I like to do around 38C. 

Remove the chilled cake from the fridge and place it onto a rotating cake stand. Pour the drip into the centre of the cake and while spinning it, use a palette knife to push the drip to the edge, gently guiding it so it flows down the side. 

Lift the cake onto your cake stand and decorate the bottom edge with chocolate curls, sprinkling some more along the top too. Allow this to sit at room temperature for a few hours before serving. If the cake is too cold, the flavour won't come through and the fillings will be firm.