I saw rainbow cupcakes in a recent Herald food section and was inspired to try it out. For some reason I forgot to follow the actual rainbow colour sequence (ROYGBIV) but they look pretty cool anyway, I thought. The oil in the recipe helps keep the cakes moist as I took them to a morning tea the following day.
I made double this recipe, which made 12 small cupcakes and 8 bigger ones.

I'm entering this into Nov's Sweet NZ, hosted by The Kitchen Maid

1 cup all purpose flour
1 1/2 tsp baking powder
1/4 tsp salt
1/2 cup sugar
2 large eggs
1/2 cup vegetable oil (or melted butter)
1/2 cup buttermilk (or yoghurt)
1 tsp vanilla extract
red, yellow, green and blue food colourings

Preheat oven to 180C. Grease a 12 cup muffin tin (or line with paper cups).
In a medium bowl, whisk together flour, baking powder and salt.
In a large bowl, whisk together sugar, eggs, vegetable oil, buttermilk and vanilla extract. Pour in dry ingredients and stir until just combined.
Divide batter evenly into 6 small bowls; each should have a little more than 1/3 cup batter (approx 6 tbsp or so for each).
Add about 1/2 tsp food coloring to each bowl to make red, yellow, green and blue batters. Mix yellow and red to make orange and red and blue to make purple. Stir well, so no streaks of plain batter remain. Add additional food colouring if necessary.
Starting with the purple batter, add a small spoonful to each of the muffin cups (just over 1/2 tbsp in each). Repeat with all remaining colours, in rainbow sequence, adding each subsequent spoonful on top of the previous colour. Do not attempt to spread the layers of colour out (as it can cause layers to combined), but allow them to spread on their own.
Bake for about 15 minutes, until a toothpick inserted into the centre comes out clean.
Cool cupcakes on a wire rack before frosting.

Quick Buttercream Frosting
1/2 cup butter, room temperature
3 tbsp milk
1 tsp vanilla extract
approx 3 cups icing sugar

In a large bowl, beat together butter, milk and vanilla. Gradually add in the sugar until frosting reaches a thick, spreadable consistency. You will probably use 2 1/2-3 cups.
Pipe or spread frosting onto cooled cupcakes.

Adapted from Baking Bites website