- Vegetable shortening like Holsum
- 3 cups all-purpose flour
- 4 teaspoons baking powder
- ½ teaspoon salt
- 1 cup unsalted butter, room temperature
- 2? cups sugar
- 5 large egg whites, room temperature
- 2 teaspoons pure vanilla essence
- 1½ cups milk, room temperature
- Red, orange, yellow, green, blue and purple gel food colouring
- Cake icing of your choice
- Preheat oven to 180°C. Brush six 22cm round cake pans (or as many cake pans as you have, reusing them as necessary) with shortening. Line the bottom of each cake pan with parchment paper. Brush again and set aside.
- In a large bowl, whisk together flour, baking powder and salt. Set aside.
- Cream together butter and sugar in an electric mixer bowl fitted with the paddle attachment. Slowly add egg whites and mix until well combined. Add vanilla and mix until fully incorporated. Add flour mixture and milk in two alternating batches, beginning with the flour and ending with the milk. Mix until well combined.
- Divide batter evenly between six medium bowls. Add enough of each hue of food colouring to each bowl, whisking until you have the desired shade. Transfer each colour to an individual cake pan. Place in the oven. Bake until a cake tester inserted into the centre of each cake comes out clean – about 15 minutes (work in batches if necessary).
- Remove cakes from the oven. Transfer to a wire rack and allow them to cool for 10 minutes. Invert cakes onto a wire rack, re-invert and leave the cakes to cool completely.
- Using a serrated knife, trim the tops of the cakes to make level. Place four strips of parchment paper around perimeter of a serving plate or Lazy Susan. Place the purple layer on the cake plate. Spread 1 cup of the icing filling over the first layer with a small offset spatula so it extends just beyond the edges. Repeat process with blue, green, yellow and orange layers.
- Place the remaining red layer on top, bottom-side up. Gently sweep away any loose crumbs with a pastry brush. Create a rainbow of different colours by using food colouring in your icing. Pipe it on with any small nozzle in alternating coloured lines, covering the top and sides. Refrigerate until set, about 30 minutes.
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