If you've ever tried to transport a cake more than a few feet, you know how difficult it can be to prevent fresh icing from smearing. Professional bakers have several ways of getting soft icing to dry to a smooth, hardened sheen. A good recipe for royal icing, which is designed specifically to dry quickly is a top baking trick. For hardening other types of icing or baked-goods frosting, time and temperature are the key factors to control.
Place the eggs whites and vanilla in the stand mixer and beat the ingredients into a froth. The recipe yields about 5 1/4 cups of icing and can be halved or doubled as needed.
Stir the confectioners' sugar in bit by bit. Then, put the mixer speed on a high setting, whipping the ingredients until they stand up. Food Network chef Alton Brown says to look for stiff, very glossy peaks, which should form after about 5 to 7 minutes. It's important to find the right consistency, or the icing could turn out runny or too stiff.
Add food colouring, if you want icing in a colour other than white.
Move the icing immediately to a pastry bag for piping, because royal icing hardens when it is exposed to the air, according to Sherry Yard, author of "The Secrets of Baking." Alternately, the icing can be kept refrigerated in an airtight storage bag for a couple of days.
Wait to harden frosting until it is fully applied to the surface of your cake or cookies. Fresh frosting goes on smooth, while even slightly dry icing is difficult to work with.
Control the temperature. Make sure your baked goods are fully cooled before applying any icing. This process can take up to several hours, but it's well worth the wait, because icing will turn gooey or even liquid on a still-warm surface. Frosting is easiest to work with at room temperature, and it will dry much more quickly this way.
Use time to your advantage. All kinds of icing harden over time, so leave it sitting out at room temperature, and you'll eventually have a firm, iced surface. This is usually a last-resort effort, because it may sacrifice the freshness of your baked goods or the aesthetics, especially if the icing hardens and cracks.
Avoid putting your finished baked goods in the refrigerator --- or, even worse, the freezer --- as a tactic to quickly dry icing. Leave them there for too long, and you'll end up having to defrost before serving.