Make the cake layers:

1. Using a mixer with a beater/paddle attachment (not a whisk), beat room temperature butter and sugar on medium speed until pale and creamy, about 2 minutes. Scrape down to the bottom of the bowl once during mixing to make sure no butter or sugar is stuck at the bottom.
2. Add eggs one at a time, mixing on the lowest speed after each addition for 30 seconds. Scrape down to the bottom of the bowl to make sure everything is incorporated before adding the next egg.
3. Add vanilla and mix for about 30 seconds to combine.
4. In a small bowl or measuring jug, mix room temperature buttermilk and oil. Add half to the butter mixture and mix on lowest speed to incorporate.
5. In a bowl, sift flour, baking soda, baking powder and salt. Add half of the flour mixture to the mixing bowl and mix on lowest speed to incorporate, scraping down to the bottom of the mixing bowl to check that batter is evenly mixed. Repeat with the remaining buttermilk mixture and then finish with the remaining flour mixture.
6. Pour half of the batter into another bowl and add a few drops of food colouring, folding it in gently with a spatula. Optionally, add a different colour to the other half of the batter or just leave it plain.
7. Grease four 6" baking pans with non-stick oil spray or butter. Divide each colour of the batter equally between two cake pans to make two cake layers of each colour.
8. Bake at 160ºC or 325ºF for 38 minutes, or until a toothpick inserted into the middle of the cake comes out clean.
9. Leave cakes to cool in their pans for 10 minutes. Use a spatula to loosen the cakes from the edges of the pans and then turn onto a wire cooling rack. Leave to cool completely.

Make the buttercream:

1. Sift the sugar to remove any lumps and check that the butter is at room temperature - you should be able to slice through it easily with a spatula.
2. In a mixer with a beater (paddle) attachment, mix butter for a few seconds until smooth. Add a quarterof the powdered sugar and salt (if using) and mix on the lowest speed until incorporated, about one minute.
3. Scrape down to the bottom of the mixing bowl with a spatula to loosen any butter and sugar and add the next quarter of powdered sugar and mix for another minute on low.
4. Scrape down to the bottom of the bowl, add another quarter of the powdered sugar, mix for one minute on low, scrape, and add the final quarter of the powdered sugar. Mix for one more minute on low, adding the vanilla once everything is mixed together and continuing to mix until it's incorporated.
5. Add the milk or cream and mix for about 30 seconds until it's combined with the buttercream. Check the consistency and add more milk or cream a tablespoon at a time until the buttercream is smooth and easily stirred.

Assemble the cake:

1. When the cake layers are cool, trim the tops and bottoms to make layers that are one inch tall/high. Use a two and four inch cookie cutter to cut each layer into three rings.
2. Spread some buttercream across the middle of the cake board. Position an outer ring of cake on the board, centering it and attaching it to the buttercream you spread. Alternate colours of cake rings to work your way in to the middle, for example pink, yellow, pink.
3. Spread a thin layer of buttercream on top of the cake as filling. Arrange the opposite colours of cake rings on top, for example yellow, pink, yellow.
4. Add another layer of filling, another layer of alternating coloured cake rings, another layer of filling, and a final layer of alternating coloured cake rings.
5. Chill the cake in the fridge for 30 minutes to set the filling and then spread a thin layer of buttercream over the cake as a crumb coat of frosting. Put the cake back into the fridge for 30 minutes to set this.
6. Frost and decorate the cake however you like!