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This is what I use to Decorate my Cookies! Dries overnight and the cookies are then stackable.
Royal Icing Recipe
6 oz (3/4 cup) of warm water
5 Tablespoons meringue powder
1 teaspoon cream of tartar
1 kilogram (2.25 lbs.) powdered icing sugar
In mixer bowl, pour in the warm water and the meringue powder. Mix it with a whisk by hand until it is frothy and thickened…about 30 seconds. Add the cream of tartar and mix for 30 seconds more. Pour in all the icing sugar at once and place the bowl on the mixer. Using the paddle attachment on the LOWEST speed, mix slowly for a full 10 minutes. Icing will get thick and creamy. Cover the bowl with a dampened tea-towel to prevent crusting and drying. Tint with food colourings or thin the icing with small amounts of warm water to reach the desired consistency.
I use the food Coloring GEL, not the liquid. This gets you the color but it doesn’t change the icing thickness.