Heat the oven to 190C/170C fan/gas 5. Butter a 25 x 20cm baking tin, and line the base with baking parchment. Stack a sheet of baking parchment over a sheet of foil, then fold in half so the parchment is facing out. Use this to divide the tin into three sections crosswise, each about 8cm wide.
Melt the butter, remove from the heat and set aside. Put the sugar and eggs in a large heatproof bowl over a pan of just simmering water, making sure the bottom of the bowl doesn’t touch the water. Beat using an electric whisk until really thick and pale, about 4-5 mins.
Add a fifth of the mixture to the melted butter and mix to combine. Pour the butter mixture into the egg mixture, and carefully fold in along with the flour. Divide the mixture between three bowls. Leave one bowl of batter plain, then fold a drop each of pink food colouring and raspberry extract into the second bowl, and a drop each of yellow food colouring and lemon extract into the third bowl.
Carefully pour one of the batters into each of the three sections in the prepared tin. Bake for 12-15 mins until cooked through, or a skewer inserted into the middle of the cakes comes out clean. Leave to cool slightly in the tin, then carefully transfer the three cakes to a wire rack and leave to cool completely.
To make the buttercream, beat the butter, sugar, vanilla and milk together for 5 mins with an electric whisk until pale and creamy. To assemble the cake, put the plain sponge on a board and spread with half the buttercream. Top with the raspberry sponge, then spread with the remaining buttercream and top with the lemon sponge. Trim the sides so they're straight, even and neat.
To make the fondant icing, sift the sugar into a bowl and mix with 1½-3 tbsp water until you have a thick, droppable icing. Transfer a quarter of the icing to a second bowl, and mix in a drop of pink food colouring. Transfer the pink icing to a piping bag.
Spread the plain fondant icing over the top of the lemon sponge, making sure it doesn't drip down the sides. Pipe thin lines of pink icing on top across the width of the cake, space apart. Drag a cocktail stick through the lines lengthways to create a marbled effect. Leave to set slightly, then slice the cake into six even rectangles.