Preheat the oven to 180 C / Gas 4. Grease and line three round sandwich cake tins.
Cream the margarine and caster sugar together until pale and fluffy. Gradually sieve a bit of the flour followed by a drop of beaten egg into the bowl and mix to combine. Repeat this process all of the eggs and all but 2 tablespoons of flour are incorporated into the cake mixture.
Make the chocolate sponge: Spoon a third of the cake mixture into a small bowl. Sieve in the cocoa powder, add the milk and mix until well combined. Tip into one of the prepared tins.
Fold remaining 2 tablespoons of flour into the unflavoured cake mixture. Put half of this mixture into another small bowl and add the vanilla extract, mixing to combine. Tip into a prepared tin.
Add strawberry milkshake syrup and the pink food colouring to the rest of the unflavoured batter and mix until a strong pink colour has been achieved. Tip into the remaining tin.
Bake the cakes in the oven for 15 minutes or until springy to the touch and a skewer inserted comes out clean (you may find as I did, that there is only enough room in the oven for two cakes on the same level at a time - this is totally fine, just bake the third sponge when the first two are cooling).
Make the buttercream icing
Place the butter in a mixing bowl and add a drop of milk. Give this a mix until the butter is nice and soft. Gradually sieve in a couple of tablespoons of icing sugar and mix into the butter with a drop of milk. Repeat this process until all of the icing sugar has been used up and the butter icing is soft and light - you might not need to use all of the milk for this. Divide the icing into three separate bowls.
Mix vanilla extract into the first bowl of icing. Into the second add the strawberry milkshake syrup and a drop of pink food colouring and beat until it reaches an even colour. Into the third bowl, sieve in the cocoa powder and add a tablespoon of milk. Mix to combine, adding more milk if the mixture is too stiff and more icing sugar if it is too loose.
Assemble the cake
One at a time slightly scrape against the sides of the cakes to reveal the colour of the sponge inside. At the same time as doing this you can ensure that you are cutting the sponges into the same shape. After trimming one cake, I like to put the second on top of it so I have an idea how much I should or should not scrape off the cake. Although the aim here is not to cut a lot off of the sponges, during this process you will naturally change the shape of them so it is important to constantly compare them so they don't end up different sizes to each other.
Spread a little of any icing onto the cake board so that the sponge cake stays in place. First, place the chocolate sponge on the board and spread with half of the chocolate buttercream, leaving a 0.5cm margin to allow for the icing to spread with the weight of the sponges on top. Next place the vanilla sponge on top and spread with half of the vanilla icing. Lastly, place the strawberry sponge on top.
Fit three piping bags with medium star nozzles and fill with the remaining buttercream icings. Use a ruler to make even markings on the sponge, scoring two vertical lines in the middle of the cake, essentially making three vertical sections on the cake (think of a tub of Neapolitan ice cream and you’ll know what I mean).
Do the outside edges first: Use the strawberry buttercream to slowly pipe around the first edge section of the cake and fill in with lines of the icing.
Line the corresponding bottom edge of the cake with dots of icing; gradually pipe one horizontal line after the other on top of the previous line of icing, until you reach the top of the cake. Repeat this process with the other edge section of the cake with chocolate icing.
Allow to dry for a couple of minutes before you do the same process with the vanilla icing in the middle section of the cake. Now your cake should be completely covered in icing. Enjoy!
If desired, dust with a little holographic edible glitter for an extra sparkle.