The best way to decorate sugar cookies is with an icing called Royal Icing, which is easy to work with and dries very solid so you can stack your cookies when storing them.
All ingredients and tools can be found at the grocery or bulk food store.
1 500-g bag icing sugar
5 tablespoons meringue powder
1/2 cup water
1 tablespoon lemon juice
You can make this in a stand mixer or using a hand mixer.
Pour the icing sugar into a bowl. Stir in the meringue powder, which is going to help the icing dry solidly. Add your water and blend for five minutes.
The icing will come together very quickly, but you must keep blending until it really starts to thicken and is very smooth and dense.
You can add flavouring if you like. Lemon is a nice contrast the sugar, but you can use vanilla or even a liqueur. Remember that vanilla will tint your icing.
Your icing at this stage will be thick enough to use for frosting an outline on the cookies. Fill a piping bag with plain white for doing this detailed work. You can use professional icing tips or just snip a small hole in the tip of the bag (if using a disposable bag).
Stand the filled piping bag in a glass with some damp paper towel at the bottom. This will keep the tip from drying out.
To make icing for filling in cookies, divide some of the white icing into bowls and add food colour. Be very careful with food colour as a little goes a long way. Add a bit of water to the icing to thin it just enough so it spreads easily.
Using your thicker white icing and piping bag you can outline the edge of your cookie or make detailed patterns on the cookie.
Using the looser icing you can then “flood” the interior of the cookie with another colour. Just add a dollop on the cookie with a spoon or use a toothpick for more control and then spread it out to the edges.
Now you can add some sprinkles or other decorations.
A neat trick to experiment with is marbling two colours of icing.
Fill two piping bags with two different colours. Then alternate horizontal stripes of colour all the way down a cookie. Using a toothpick, drag a line through the icing vertically through your strips. Repeat across the whole cookie.
You can also drop dots of colour onto a white background and drag the toothpick through the dot to make a heart shape or swirl.
If you just want to add sprinkles onto a cookie but no icing, simple slightly wet a pastry brush with water and sweep it lightly across the top of the cookie (obviously you don’t want to soak it). Then sprinkle your coloured sugar on top and it will stick once dry.
Depending on how much icing you’ve used, these cookies could take up to eight hours to dry. Be sure the icing is completely solid before storing.