Ingredients

• 3 x 250g pkts unsalted butter, at room temperature
• 465g (2 1/4 cups) caster sugar
• 9 eggs
• 3 teaspoons vanilla extract
• 675g (4 1/2 cups) plain flour
• 450g (3 cups) self-raising flour
• 750ml (3 cups) milk
• 6 different food colourings (see note)
• 2 x 250g pkts unsalted butter, at room temperature
• 750g (5 cups) icing sugar mixture
• 125ml (1 ⁄2 cup) milk

Method

• Step 1 Preheat oven to 180°C/160°C fan forced. Grease and line two 20cm (base measurement) round cake pans with baking paper.
• Step 2 To make 2 of the cake layers, use electric beaters to beat 250g butter and 155g (3 ⁄4 cup) caster sugar in a bowl until pale and creamy. Beat in 3 eggs, one at a time, until well combined. Beat in 1 teaspoon vanilla extract. Fold in 225g (1 1/2 cups) plain flour, 150g (1 cup) self-raising flour and 250ml (1 cup) milk, in alternating batches, until well combined.
• Step 3 Divide the mixture evenly between 2 bowls. Add a different food colouring to each bowl, adding a few drops at a time and stirring until the mixture is vibrant in colour. Spoon the coloured mixture from each bowl into separate lined pans. Bake for 25 minutes or until a skewer inserted into the centre comes out clean. Set aside for 10 minutes to cool slightly before transferring to a wire rack to cool completely.
• Step 4 Repeat steps 2 and 3 twice with the remaining butter, caster sugar, eggs, vanilla, plain flour, self-raising flour, milk and 4 different food colourings, to make 4 more cakes.
• Step 5 For the rainbow icing, use electric beaters to beat the butter and icing sugar in a bowl until pale and creamy. Beat in the milk. Divide the mixture evenly between 2 bowls.
• Step 6 Use a large serrated knife to trim the domed tops off the cakes. Place 1 cake, cut-side down, on a platter. Using 1 portion of the icing, spread a thin layer of the icing over the top the cake. Top with another cake and spread a thin layer of icing over the top. Continue layering with the cakes and icing, finishing with a layer of cake.
• Step 7 Divide remaining portion of icing among 6 bowls. Add a different food colouring to each bowl to match colours of the cakes. Starting from the bottom cake layer, and matching the colours of the cake with icing, spread coloured icing over the side of each cake layer to cover. Cover the top cake layer with the icing colour to match.