Runny frosting can easily ruin a beautifully baked cake. If your frosting's consistency is "off," your decorations will follow suit. The process is delicate: Just a few drops of liquid can skew results, as can subtle changes in humidity, temperature and ingredients. If your frosting looks limp, don't lose heart; with the right tools, you can easily correct it.
Determine the Proper Stiffness
Spreadable frosting should be of a consistency that keeps its shape on a lifted frosting knife without sliding off, but it's too stiff if it doesn't spread effortlessly.
Frosting for decorating must be stiffer. Also referred to as "icing," this type of frosting is used to sculpt flowers, figure piping and strings. If this type of icing is not prepared to a sufficient stiffness, the decorations will flop over and run, but don't overdo it. An over-stiffened mixture will crack when squeezed through the decorating tool.
Balance Liquid and Powdered Sugar
Adjust the proportion of liquid to powdered sugar. To stiffen "limp" frosting, add powdered sugar by the tablespoon, whipping thoroughly as you go. Using a stand mixer makes this job much easier than doing it by hand. Fully incorporated each tablespoon before you add the next one, as consistency changes significantly with small adjustments.
If you accidentally over-thicken your frosting, beat in small amounts of additional liquid. When adding liquid, work just one teaspoon at a time, incorporating each completely before adding another.
If your frosting started out at the perfect consistency but softens on the counter, it has likely become too warm. Put it in the refrigerator for a few minutes to correct the problem.
If your frosting recipe starts out warm -- for instance, a frosting based on freshly melted chocolate -- refrigerate the batch overnight. Bring it to room temperature the next day, then thoroughly whip it again and adjust the balance of sugar and liquid from there.
Working With Royal Icing
Royal icing recipes need special care. If you integrate more than half a cup of powdered sugar to thicken your icing, you must also add 1 or 2 teaspoons of meringue powder. The powder -- essentially, powdered egg whites -- will help the delicate frosting to stay light and keep its shape after the introduction of the proportionally "heavy" sugar.