Heat the oven to 180c/fan 160c/gas 4. Grease and line two 8 inch, deep cake tins.
Beat the butter, sugar and lemon zest together until light and fluffy. Gradually add the beaten egg, a little at a time, beating well in between each addition.
Gently fold in the sifted flour and salt, adding enough undiluted cordial (up to 60ml) to create a dropping consistency ( the mixture should drop off a spoon when lightly tapped).
Divide the mixture between the two tins (weigh each tin to easily judge roughly the same amount of mix). Place the tins on the middle shelf of the oven and bake for 25 minutes. Don't be tempted to open the door while they're cooking, but after 20 minutes do look through the door to check them.
Using a pastry brush, spread the elderflower and lemon syrup on each cake layer.
Make the buttercream icing by whisking in the bowl of a stand mixer the butter, mascarpone, double cream and icing sugar together until thick and smooth.
Meanwhile, sift your icing sugar.
With the mixer running on medium speed, add your icing sugar 20g (2 tbs) at a time until you have incorporated 250g of it. Stop the mixer, scrape down the bowl, and taste your buttercream. Continue adding sugar slowly until it is at the sweetness you prefer. More sugar will create a more stiff buttercream, but it will still hold its shape well with only 250g of sugar. Add the elderflower cordial or spare drizzle and mix to taste.