1. Preheat your oven to 180C/160C fan/gas 4 and line a muffin tin with 6-9 large cupcake cases.
2. Cream together the butter and sugar in the bowl of your stand mixer until super pale and fluffy.
3. Add the eggs, one at a time, mixing thoroughly.
4. Finally, fold through the flour, oil and vanilla bean paste until just combined.
5. Divide the mixture evenly between four bowls (or as many colours as you are using) and add a small amount of gel food colouring to each bowl. Stir through until the mixture is an even colour.
6. Spoon the mixes into a large piping bag, doing your best to keep them separate and then pipe the mixture into the cupcake cases, filling them around ¾ full.
7. Bake your cupcakes in the preheated oven for around 17-18 minutes, until a skewer inserted in the centre comes out clean.
8. Allow to cool in the tin for around ten minutes, before transferring to a cooling rack to cool completely.
7. To make the buttercream, begin by creaming your butter until very pale and smooth. This may take a while, but the longer you leave it, the better.
8. Add in the icing sugar, a little at a time, ensuring each addition is fully incorporated before adding the next.
9. Add the milk and mix for a further two minutes.
12. Next, use a piping bag with your favourite tip to pipe a swirl of buttercream onto each cupcake, before topping with your favourite rainbow sprinkles.
13. Once the buttercream has set, store your cupcakes in an airtight container for up to three days at room temperature.