• Preheat the oven to 160C/320F/Gas mark 3. Start by greasing 6 non-stick 6" cake tins with butter or oil and line them with baking paper if necessary.
• Next, add 700g of room temperature butter to a large bowl.
• Add 700g of sifted caster sugar.
• Whisk for 3-5 minutes or until pale yellow.
• Add 1 teaspoon of vanilla extract to the mixture. Then measure 700g of room temperature eggs. Add one into the bowl and whisk until the egg is no longer visible. Repeat the previous step for the remaining eggs. The mixture should look smooth and silky.
• Add 350g of sifted self-raising flour to the mixture and 2-3 tablespoon of room temperature milk and mix. Repeat the previous step with the remaining 350g of self-raising flour and milk then mix.
• You will need red, orange, yellow, green, blue and violet food coloring. Add equal amounts of the cake batter to six different bowls and add ⅛-1/4 teaspoon of each food coloring and mix until well combined.
• Add each colored cake batter into the greased 6" tins and bake for 30 minutes or until done. To test if your cake is ready stick a knife into the cake. If it comes out clean then it is done but if it is wet with cake batter then it's not ready. Once the cakes are ready leave them to cool completely on a cooling rack before decorating.

How to Make Buttercream

• Add 900 g of room temperature unsalted butter to a large bowl and whisk on a low speed for 1 minute. Then add 850 g of the sifted icing sugar and whisk on a medium speed for 3 minutes.
• Add the remaining 850 g of sifted icing sugar, 2 tsp of vanilla extract, and ½ to ¾ cup of double cream. Whisk for 3-5 minutes or until spreadable.

How to Assemble the Rainbow Cake

• Make sure you stack the cakes in the correctorder once it's time to decorate.
• Add buttercream ½ a tablespoon of buttercream to the center of an 8" cake board. Then place the violet cake on the board. Option to add homemade syrup to the cake to keep it moist. (see notes). Then add buttercream. Repeat this step for the rest of the cakes.
• Then add buttercream to a piping bag and cover the cake in buttercream. Use a smoother to smooth the cake. Then refrigerate the cake for 30 minutes.
• Next, you have the option to add a second layer of buttercream or skip this and add sprinkles to the entire body of the cake using your hands or a spoon.
• Finally, add a star nozzle to a piping bag, add buttercream and add swirls around the rim of the cake.
• Now get ready to serve this show-stopping rainbow birthday cake with sprinkles! Cut into the cake, take your first slice and admire the beautiful rainbow colors!