Cookie decorating goes back to the Middle Ages, and is a sophisticated art. Once limited only to certain guilds, today anyone can learn its secrets. One of the most intriguing effects in cake and cookie decorating is the use of cake decorating or pastry dusts. Also known as edible glitter or edible dust, they are used as an easy way of putting a classy, fairylike or romantic finish to your baked creations.
There are several basic kinds of pastry dust. You can see examples at sugarcraft.com, globalsugarart.com, thebakerskitchen.net, and lusterdust.com. If you do rubber stamp art with embossing powder, or work with tempera powder, you'll see that there are similarities. Petal dust has a matte finish; pearl dust, naturally, looks pearlescent. Sparkle dust is coarser and appears more like glitter. Highlighter dust has an extremely metallic look and can be used alone or with the others. Luster dust has a metallic finish and somewhat softer colors. It is great for weddings, Bar or Bat Mitzvahs, and other splendid occasions where things should look "gilded".
Pastry dusts are appropriately used on fondants, sugar and gum paste decorations, cookies and iced cakes. The most familiar brands are certified Kosher.
Luster dust, like all pastry dusts, comes in vials of two to four grams; you use only a quarter teaspoon or less at a time. You can use it right out of the vial or mix it with something evaporable if you want to paint it on. You can buy it in sets which include special mixing cups and brushes. Or, you can just use a small jar lid and a clean paintbrush. Take a tiny amount of dust from the vial, add a few drops of Everclear or vodka, vanilla or lemon extract (don't use water), and stir well. Some people simply dip a brush into the dust and put it in the mixing cup, then dip the same brush into the liquid and mix tiny bits at a time. Remember -- a little of this stuff goes a long way! You can experiment with blending colors and even types of dust as you go along.
Where can luster dust be used? You'll be amazed. If you're making cookies without icing, brush them with a little egg white and sprinkle dry dust over them before baking. You can also sprinkle it dry over fondant. Cookies and cupcakes with icing or gum paste decorations can be painted in fancy designs after the icing has dried thoroughly. Some people find luster dust more effective if they put on two coats. You can also try highlighter dust for the second coat for added sheen. Luster dust is especially effective with royal icing. Try decorating cakes with luster dust in stenciled patterns, or use subtle, tiny amounts as highlights on sugar flowers.
Pastry dusts should not be mixed into fondant or icing; they won't show unless they are on the surface. Be careful when brushing dusts on chocolate - it's best to use a natural paintbrush so it won't scratch.
You can learn a lot more about how to use pastry dusts by taking pastry classes or watching cooking shows. There are several YouTube videos showing both simple and complex techniques. Valerie Peterson's book "Cookie Craft" is another great resource. Have fun!