For the cake: Firstly preheat your oven to Gas Mark 4/180C and grease and line three 8" baking tins and set aside. Then using a bain-marie, melt the white chocolate over a low heat and then set aside to cool.
Sift together all the dry ingredients and set aside. Then, place the butter in a bowl and beat with an electric mixer on a high speed until creamy and light in colour. Add the sugar and continue to mix until fluffy.
On a medium speed add the egg yolks, vanilla and melted white chocolate. Then with the mixer on low, alternate adding in the dry ingredients and milk in three batches, ending and starting with the dry ingredients.
Mix until just combined, then pour into the prepared tins. Bake for 20-22 minutes until golden. Leave to cool a little in the tins, and then put them on a cooling rack to completely cool.
For the buttercream flowers: Whisk together the sugar and egg whites in heat-safe bowl until combined. Then using a bain-marie, heat the mixture, stirring occasionally until it reaches 155-160F on a candy thermometer. The mixture will be very hot and the sugar will have dissolved. Once the mixture has reached the right temperature, take the bowl off the heat and using an electric mixer whisk the mixture on high speed for about 10 minutes until you reach medium-stiff peaks and the mixture has returned to room temperature.
With the mixer on low, add in the butter a tablespoon at a time ensuring each piece is fully incorporated before adding the next. The butter must be at room temperature for it to be properly incorporated. Once the butter has been mixed in, add the vanilla extract and mix again for a few minutes until the buttercream is silky smooth and light.
Divide the buttercream into different bowls according to how many different flower/leaf colours you want. Then mix food colouring into each and place each into piping bags. Sset aside the leaf buttercream to use later. Then using a flower nail and piping tips create an array of flowers, on squares of baking paper. Place the squares onto baking trays and place in the freezer while you make the rest.
For the cake buttercream: Follow the swiss meringue buttercream recipe for the flowers, up to and including adding the butter in. Strain the raspberries through a sieve into a separate small bowl, so you are left with the puree without the seeds.Mix in the sugar and lemon juice, and then pour the mixture into the buttercream.
Mix this on low at first, and then gradually increase the speed. It may take a few minutes for the puree to combine with the buttercream, so keep mixing until its fully combined.
For assembly: Place one sponge onto the cake board and spread with a layer of raspberry buttercream and half of the freeze-dried raspberries. Then repeat with the second sponge layer. Then place the final sponge layer on top. Crumb coat the cake with a thin layer of the raspberry buttercream and then place in the fridge for 20 minutes to set.
Then cover the chilled cake with a thick layer of raspberry buttercream and using a bench scraper, scrape off the excess to leave a smooth exterior.
Place some of the raspberry buttercream into a piping bag and pipe a ring on top of the cake for the flowers to sit on. This gives the flowers some height, rather than all lying flat on top of the cake. Remove the buttercream flowers from the freezer, and peeling off the baking paper squares arrange them on the cake. Finally, pipe leaves wherever needed on the cake for the finishing touch.