Preheat the oven to 170C/160C Fan/Gas 3. Line two 12-hole cupcake trays with cupcake cases.
For the cupcakes
beat the flour
sugar
baking powder
salt and butter together in a freestanding mixer with a paddle attachment (or use a handheld electric whisk). Mix in half the milk until it is just incorporated.
In a separate bowl
whisk the eggs
vanilla extract and remaining milk together. Pour into the flour mixture and beat until the mixture is smooth.
Divide the batter equally between four bowls and colour each one with food colouring
so you end up with four brightly coloured bowls of cake mixture.
Now layer the coloured mixtures in the cupcake cases. Starting with one colour
divide the mixture equally between the cupcake cases and smooth it out so you have a layer at the bottom. Continue with all the other colours in the same way
evenly spreading each layer out and being careful not to mix the colours together
so that you end up with distinct layers of mixture.
Carefully transfer to the oven and bake for 20–25 minutes
or until the sponge bounces back when touched and a skewer inserted into the centre comes out clean.
Leave the cupcakes to cool slightly in the tray before turning out onto a cooling rack to cool completely.
For the vanilla buttercream
beat the butter with a handheld electric mixer until soft. Sift in the icing sugar and incorporate to make a smooth icing. Add the vanilla extract and beat the icing for a further few minutes until light and fluffy. Transfer to a piping bag fitted with a large star nozzle.
When the cupcakes are cold
pipe the vanilla buttercream on top of the cakes and decorate with the hundreds and thousands. To finish
