Preheat the oven to 190C/170C fan/gas 5. Grease and line the base with parchment of the baking tins. See Recipe Note No. 4 for more tips on the baking tins.
Melt butter, set aside. Whisk sugar and eggs over simmering water until thick. Combine melted butter with egg mixture, then fold in flour. Divide batter into three bowls, leaving one plain and adding pink and raspberry extract to one, and yellow and lemon extract to another.
Pour batters into sections in the tin. Bake for 12-15 mins until cooked through. Cool in the tin before transferring to a wire rack.
Beat butter, icing sugar, vanilla, and milk until creamy. Assemble the cake by spreading half the buttercream over the plain sponge, topping with raspberry sponge, remaining buttercream, and lemon sponge.
Mix fondant icing sugar with water until thick. First start with 2 tablespoons and only add another tablespoon if the consistency is still too thick. Optionally add pink food colouring to a portion and transfer to a piping bag.
Spread plain fondant icing over the lemon sponge. If you're decorating the top with pink icing, pipe thin lines of pink icing across the cake and drag a cocktail stick through them lengthways for a marbled effect. Decorate with the biscuits. Allow to set slightly before slicing.