Preheat the oven to 160C (140C fan). Lightly grease 3x 20 cm cake sandwich tins and line the base of each with baking paper. Set aside.
Using an electric mixer, beat the butter and the sugar together until pale and fluffy, about 3 minutes. In a small jug, whisk together the eggs, soured cream and elderflower cordial.
Add the flour to the egg and butter mixture, followed by the eggs mixture. Mix to combine. Add the lemon zest and mix everything together until you have smooth batter.
Divide the batter between the cake tins and smooth the top with back of the spoon or spatula. Bake the sponges for 35- 40 minutes, until golden brown and springy to the touch.
Remove from the oven and allow to cool in the tins for 10 minutes. Remove from the tins and transfer to the cooling rack to cool completely.
Once the sponges have cooled completely, trim the tops using a serrated knife or a cake leveler (if needed).
To make the syrup:
Place all of the ingredients in a small saucepan over a medium heat. Allow the sugar to dissolve completely, then bring to boil for couple of minutes.
Remove from the heat and allow to cool.
Using a skewer or a toothpick, poke the sponges all over the surface. Brush the cooled syrup over each sponge (you may not need to use all of the syrup).
To make the buttercream:
Place the softened butter into a bowl of an electric mixer and beat until pale and fluffy, about 3 minutes.
Add the icing sugar in 3 additions, beating well after each addition.
Pour the elderflower cordial and beat for another 3 minutes until you have smooth and fluffy buttercream.
To make the white chocolate ganache drip:
Finely chop the white chocolate and place in a heatproof bowl.
Place the double cream and butter in a small saucepan and cook until butter have melted, and the cream is hot (do not boil!).
Slowly pour the hot cream and butter mixture over the white chocolate and leave it to stand for couple of minutes. Stir gently to combine until you have smooth, shiny ganache.
Leave the ganache to cool for 10 – 15 minutes, allowing it to thicken. Transfer to a piping bag fitted with small plain nozzle (0.5cm opening will work well).
To assemble the cake:
Spread a small amount of buttercream in the middle of the cake board/ plate and place the first sponge on top of it.
Add about 3 Tbsp of buttercream on top of the sponge and spread in an even layer. Add an even layer of lemon curd on top of the buttercream, then place the second sponge on top and repeat with buttercream and lemon curd.
Place the third sponge on top. Use the remaining buttercream to cover the sides and the top of the cake. Save any leftover buttercream and transfer to a piping bag fitted with any decorative nozzle (optional).
Pipe the white chocolate ganache drip into the very edge of the cake, allowing it to set.
Pipe any leftover buttercream on top of the cake.
The sponges can be made up to 3 days in advance. Wrap the cooled sponges in a double layer of clingfilm and store in the fridge.
Buttercream can also be made in advance, 1-2 days ahead. Store it in the fridge, then bring to room temperature before using it. You may want to beat it briefly with spoon or spatula before using.
Decorate the cake with anything that your heart desire. Slices of lemon will work, fresh berries will also look and taste wonderful. You can also use sprinkles, mini-meringues or other decorations.