A step-by-step tutorial from our Cake Decorating School... Rainbow Cookies!
Learn from expert Tammy Youngerwood and create these colourful rainbow cookies. This ‘wet-on-wet’ piping technique is easy to do and produces a stunning result! Makes enough to decorate around 36 7.5cm (3 inch) cookies!
Ingredients
For the royal icing • 5 tbsp dried egg white/meringue powder (I prefer Wilton’s Meringue Powder) • 1 tsp cream of tartar (optional) • 175ml (6fl oz) warm water • 500g (1lb 2oz) icing sugar, plus an extra 250g (9oz) if necessary • food colouring, red, orange, yellow, green, blue, violet 1. In a clear glass bowl, mix the warm water and egg white powder. Use a handheld whisk or fork to mix the powder fully in the water. Mix for about 1-2 minutes until you see lots of little bubbles. Look from the bottom to make sure it is all getting mixed in. 2. (Optional) Add the cream of tartar and mix for another minute. 3. Pour the mixture into the bowl of an electric mixer fitted with a paddle or K attachment and add the icing sugar one cup at a time. Mix on low to incorporate the sugar, then increase the speed. 4. Whisk on high for 5 minutes – the mixture should become opaque and very thick. If the mixture is still runny, add another cup of icing sugar and whisk for another 2 minutes. 5. Store in an airtight container. This keeps in the fridge for about 1 week and in the freezer for 3-4 months. Allow to come to room temperature and whisk again before using. 
How to use the royal icing 1. Thin to ‘20-second icing’ consistency – this means when you drag a knife through the icing and count to 20 seconds, the icing should flow back together so no line can be seen. If the icing is too stiff, add a few drops of water, mix and test again. Tint to the colours you need to fill the piping bags. The 20-second icing is thick enough to create a dam around the cookie edge, but loose enough to flow together and create a smooth finish. 2. Fill 6 bags with the colours of the rainbow (I omitted Indigo), and 1 bag with white icing. The quantity will vary depending on how many cookies you are making. TIP: Royal icing dries when exposed to air, so always keep covered in a plastic tub when not in use.
Decorating the cookies
Using your favourite plain cookie recipes, roll, cut and bake your rainbow cookies. Allow to cool before icing. Step 1: Wet-on-wet technique Beginning at the top, line the rainbow with red royal icing. Do not allow the icing to dry, but follow quickly after with each of the subsequent colours of the rainbow. 
The colours should touch each other to create a smooth finish. This is called the 'wet-on-wet technique'; when wet colours are piped next to each other they will dry with a seamless finish. 
Step 2 Add the clouds Add several dots of white, not touching the rainbow. Be generous with the icing so the white icing becomes a little puffy. Allow it to dry for a minimum of 30 minutes (or until the icing has begun to crust). Step 3 Add more clouds - wet on dry The first white dots and the rainbow should be crusting over now, so go and add several more full dots to each cloud. Allow to dry. Add the remaining white dots wherever needed to complete the look of the fluffy clouds. Allow to dry for 6 hours or overnight before packaging.
