For the Italian meringue buttercream, whisk the egg whites in a bowl until soft foamy peaks form. Weigh out 25g/1oz of the caster sugar, then continue to whisk the egg whites and gradually add the sugar until well combined. Continue to whisk until stiff peaks form when the whisk is removed.
Heat the remaining sugar and the water into a small saucepan on a low to medium heat until the mixture looks clear. (Swirl the pan if some sugar remains undissolved.) Increase the heat to just above medium and boil until the sugar mixture reaches 118C/245F. (Measure this with a sugar thermometer.)
Continue to whisk the egg whites and slowly add the sugar syrup, continue whisking until the bowl no longer feels hot then add the butter, at a little at a time, until fully incorporated. Continue whisking for a further 2-3 minutes, or until smooth and thick.
Beat the buttercream icing with a wooden spoon to remove some of the air then spoon half the mixture into an icing bag fitted with a small plain nozzle. Spoon the remaining buttercream into a bowl and cover with cling film.
To make the sponges, preheat the oven to 190C/375F/Gas 5. Line three 30cm x 23cm/12in x 9in Swiss roll tins with greaseproof paper.
For the pink sponge, weigh the eggs in their shells then weigh out equal quantities of flour, sugar and margarine. Place all the ingredients into a bowl, beat until light and creamy then transfer the mixture to the tin and level the surface with a palette knife. Bake in the oven for 15-20 minutes, or until the sponge is golden-brown and cooked through. Allow the sponge to cool then turn out onto a board and peel off the paper.
For the vanilla sponge, repeat step 6, adding the vanilla seeds instead of the food colouring.
For the chocolate sponge, repeat step 6, substituting two tablespoons of the flour for the cocoa powder.
To make the mini brandy snap ‘flakes', preheat the oven to 180C/350F/Gas 4. Line two baking trays with greaseproof paper.
Heat the sugar, butter and golden syrup in a saucepan until the sugar has dissolved. Stir in the flour, brandy, ground ginger and lemon zest until well combined then spoon small spoonfuls onto the baking trays, leaving plenty of room for the mixture to spread. Bake for 6–7 minutes, or until golden-brown and holes are starting to develop. Remove from the oven, allow to cool a little then wrap each one around an oiled pencil. Set aside to cool on a wire rack. (If the brandy snaps become too stiff to work, return them to the oven for a few seconds.)
Dip the ends of the brandy snaps into the melted chocolate and set aside to cool on a wire rack.
To make the ganache, heat the double cream in a saucepan until just boiling, then add the chocolate. Beat until thick and smooth then remove the pan from the heat and set aside.
To make the raspberry sauce, push the raspberries through a fine sieve and stir in the icing sugar to make a thick paste. Pour the sauce into a squeezy bottle.
To assemble, using a serrated knife and a ruler, cut the sponges into 5cm/2in squares, trimming if necessary to ensure they are all of the same thickness. Sandwich the cakes with the buttercream; pink on the bottom, vanilla in the middle and chocolate on the top. (Refill the piping bag with buttercream if necessary.) Spread ganache on top of the cakes and top with a swirl of buttercream. Finish with a brandy snap ‘flake’ and drizzle over the raspberry sauce.