Preheat the oven to 170°C/fan 150°C/gas mark 3½. Grease and line two 8 inch cake tins with greaseproof paper.
In a large bowl (or bowl of a stand mixer) cream together the butter and sugar until very light and fluffy. Add in the eggs one at a time, beating well between each addition. Add a small amount of the flour if the mix begins to curdle.
Add the vanilla and salt and mix until well incorporated, then add half the flour and all the baking powder and mix again until just combined. Now add the milk, citrus juice, zest and remaining flour and mix, then scrape down the sides of the bowl and mix for a final time until you have a smooth and creamy batter.
Divide the batter between the two cake tins and bake for 25-30 minutes or until the cakes spring back when touched (or a skewer comes out clean when inserted).
Once baked, leave the cakes in the tins for 10 minutes then turn out and leave to cool completely on a wire rack.
Meanwhile, make the floral citrus syrup. To do this, just put the sugar and water in a small saucepan and heat gently until the sugar dissolves. Set aside to cool a little for 5 minutes then stir in the remaining ingredients.
For the buttercream place all the ingredients in a large bowl (or bowl of a stand mixer) and beat with electric beaters on a low speed until the icing sugar is well incorporated (it’s a good idea to cover the bowl with a clean tea towel at this stage to avoid an icing sugar cloud!).
Once the icing sugar is incorporated, turn up the speed and beat for three minutes or until very light, smooth and creamy. Set aside.
Once all the components are completely cool you can assemble the cake. To do this drizzle half the syrup over the surface of each sponge and allow it to sink in.
Divide the buttercream into three. Spread a third over the first sponge, then sandwich on the next and spread over another third. Use the remaining buttercream to rustically cover the sides of the cake.
Decorate with multi-coloured hundreds and thousands.
This cake will keep in an airtight container in a cool part of your kitchen or the fridge for three days.