Put your aquafaba into your bowl or stand mixer and beat on high for several minutes until light and frothy.
Add 5 cups of your powdered sugar (sifting it first helps remove clumps that may clog up your icing nozzle) to your frothy aquafaba and mix on medium speed until smooth and glossy.
Add in your corn syrup and the other 1.5 cups of powdered sugar and beat again until well incorporated. If you don’t want to use the corn syrup, you can skip it, but it helps to make the icing dry shinier and be a little more smooth for decorating.
Now, once your icing is smooth and glossy, check the consistency to make sure it’s thick enough for any outlining you may want to do. I feel like this vegan icing needs to be a little thicker than you would normally make an egg-based icing for outlining, so if you’ve made egg-based royal icing before, try and shoot for a little thicker than usual. If you pull up the beater on your mixer, you want the icing to make a peak that will droop and almost drop, but will hold in place on the beater. That’s usually a good outline consistency for me.
Also, since I make it a little thicker, I would suggest using piping bags and metal tips for outlining as thicker icing can sometimes be a struggle to get out of icing bottles and you can more easily push the icing out of a bag you are squeezing than a bottle.
Add your thicker outline icing to as many bowls as you need outline colors and add your coloring to desired shade. To thin the icing for a flood color, I like to add my gel coloring (a little goes a long way!!) and then use fresh lemon juice and just squeeze a few drops in until I get a good consistency.
Again, I find I need to make this icing a little thicker than egg-based royal icing or it can run off the sides. So I add lemon until the icing ribbons off my mixing spoon at a smooth, but not super fast, pace. It’s also pretty subjective what consistency you like to decorate with, so if you’re new to icing, give it your best shot and you can always add more powdered sugar to thicken up icing and more lemon juice or water to thin it out. I like to use icing bottles for flood colors and they are a lot easier to pour into than a bag too.
Once you have your colors mixed, it’s time to ice your cookies! The other difference with this vegan icing is that it takes a longer time to set, so if you are doing an outline that you want to be totally distinct (or icing a color on top of another that you want to stand out and not sink in to the color below), you really need to give the icing several hours to set first before moving on to the next step (putting it under a fan can speed up dry time and make your cookie shinier, though!).
I would wait at least 24 hours before stacking iced cookies as well, so if you need to transport them before then, carry them flat in a box in a single layer.