1. Preheat the oven to 160°C. Lightly grease two 20cm sandwich tins and line the bottom of each with baking paper. Set aside.
2. Using an electric mixer, cream the butter and sugar until pale and fluffy, about 2 minutes. Add Lyle’s Golden Syrup and eggs and beat well (don’t worry if the mixture curdles a bit at this stage).
3. Sift in the flour and baking powder and stir to combine. Add the lemon zest, buttermilk, and vanilla and mix again. Divide the batter between the baking tins and smooth the tops with the back of the spoon or spatula. Bake for 20-25 minutes, until golden brown and a toothpick inserted in the middle, comes out clean. Remove from the oven.
4. Make the lemon drizzle by placing all the ingredients in a small saucepan. Stir to combine and allow to simmer for 5-8 minutes, until all the sugar has melted and the mixture is syrupy. Remove from the heat and allow it to cool for 5 minutes.
5. Pour the lemon drizzle over baked sponges and allow them to cool completely.
6. Prepare the buttercream. Place the butter and soft cheese in a mixing bowl and beat for 2-3 minutes until well combined. Add the icing sugar, lemon juice, vanilla, and salt and mix until you have a smooth buttercream. If the mixture feels too runny, add more icing sugar, if it is too stiff, add a little bit more lemon juice.
7. Transfer one of the cooled sponges to a cake stand or a serving plate. Spread half of the buttercream over the top of it and place the second sponge on top. Spread the remaining buttercream over the top of the cake.
8. Optionally, top the cake with some more lemon zest and some whipped cream and lemon slices.