I recommend using royal icing for decorated cookies if you want to hand-pipe designs that stay in tact, since the icing dries hard.
3/4 cup warm water
5 tbsp Meringue Powder
1 tsp vanilla extract (or clear imitation vanilla extract for white icing)
1 kg bag of icing sugar
1) Add meringue powder to the mixer bowl. Add water and whisk until dissolved.
2) Add in vanilla extract and whisk for 30 seconds.
3) Add all icing sugar at once and mix on low with an electric mixer fitted with the paddle attachment. (Using the whisk attachment will add too much air).
4) Mix on speed 2 for 2 minutes. Scrape sides of bowl with a spatula.
5) Continue to mix on speed 2 for 3 more minutes, until the icing becomes an opaque white colour and reaches thick consistency. When removing the paddle from the bowl, the icing should hold shape and have stiff peaks.
To achieve a thinner consistency icing, add a few drops of water at a time and mix well. To test your consistency, use a knife to draw a line through the surface of your icing:
THICK consistency icing should hold shape. (Eg. To pipe rosettes, leaves)
MEDIUM consistency icing should flatten itself in 13-20 seconds. (Eg. for lettering)
FLOOD consistency icing should flatten itself in 8-12 seconds. (For the base of the cookie - should have a smooth surface).
Wait 8 - 12 hours before handling cookies or adding a second layer, to allow enough time for the royal icing to dry.