- Preheat the oven to 350°F (180°C). Line two 12-cup muffin pans with 15 pink paper or foil liners.
- Sift the flour, sugar, baking powder and baking soda into a large mixing bowl. Add the lemon zest and whisk in, to draw out the lemon flavour.
- In a separate bowl, whisk the oil, buttermilk, eggs, egg yolk, lemon juice and vanilla together. Add this all at once to the flour and whisk vigorously by hand until the mixture is smooth and thickened, about 2 minutes.
- Scoop the batter into the muffin tin, filling then about 2/3 full. Bake for 15 to 18 minutes, until they spring back when gently pressed. Cool the cupcakes in their tin on a rack for 15 minutes, before removing to cool completely.
- For the frosting, use electric beaters or a stand mixer fitted with the paddle attachment, beat the butter at high speed for 2 minutes, until fluffy. Add half of the icing sugar and beat, starting at low speed and then increasing to high, stopping to scrape the bowl once or twice. Add the remaining icing sugar, lemon zest and salt, and beat again at low speed and then increase to high, scraping the bowl once or twice. Add the cream, lemon juice and vanilla and beat at low speed and then increase to high. Beat for 3 to 4 minutes until fluffy and light and to build structure so your frosting will hold its shape.
- Divide the frosting between 3 bowls. Stir pink food colouring into one bowl, yellow into a second and leave the third white. Spoon each of the frosting into their own, separate small piping bag (no tip needed). Lay out a square of plastic wrap. Snip a ¾-inch (1.7 cm) opening at the end of each piping bag and pipe a wide strip of each colour frosting, side-by-side, the full length of the plastic wrap at the centre . Repeat this one more time, so that the frosting ‘ribbons’ are 6 in a row, or alternating colours. Have ready a large piping bag fitted with a large star tip. Roll up the plastic wrap so that the ribbons of frosting retain their shape and carefully lift this and place it into the large piping bag, with an open end of the plastic inserted towards the tip. Leave the plastic wrap in the piping bag.
- Pipe large spirals of the frosting onto each cupcake – you will see that you get a colourful rainbow swirl of frosting. Top each cupcake with sprinkles or candies and enjoy.