It's a frustrating situation: You're ready to decorate a beautiful cake, but when you dip a knife into the batch of icing, it comes away limp. Runny icing won't form into fetching borders, decorative accents or pretty scrollwork, so if your plans for dessert include any of those thrills, you'll have some work to do. As long as you understand what happened to your icing, you can use a trick or two to fix it -- or just enjoy it as it is.
What to Expect
If you set upon your cake -- or any baking project -- with icing that doesn't hold its shape, don't expect a pillowy puff of firm buttercream. Runny icing will act much like a glaze, soaking into the cake wherever the layer is thin. If it's sufficiently liquid, frosting may soak entirely through and pool under the cake.
Runny icing can not be used for piping decorations. Test piping icing by "drawing" on paper with your decorating tools; if it bleeds or runs, it's not stiff enough.
Take Care With Temperature
If you start to frost and you notice that the icing is too runny to achieve your desired result, try playing with temperature. Ideally, your ingredients should start out at room temperature; if the oven has warmed up the room, you'll need to cool down the materials you're working with. Transfer your frosting into a metal mixing bowl to more efficiently conduct temperature. Then, either put the bowl into an ice bath or into the freezer for a few minutes until the consistency firms up.
Take Special Care With Cream Cheese
Cream cheese icing is especially tricky. The liquid in the cheese separates from its fat during mixing, dissolving the powdered sugar in a way that doesn't regain its firmness even with vigorous re-whipping or with that old standby of frosting-correction: the addition of more powdered sugar. If you notice that your cream cheese frosting has become runny when you start to work with it, scrape the cake clean and set it in a cool, dry place while you make new icing. When you do, use a stabler recipe: ideally, full-fat cream cheese in a recipe that uses ample butter. Then, carefully avoid over-beating.
What to Do With Runny Icing
If you're stuck with a batch of runny frosting that doesn't complement your current baking project, all is not lost. Pour it into an airtight container and store it in the freezer. Defrost it when you're ready to drizzle the icing over cinnamon buns, brush it over cookies or set scones off with a fetching garnish.
"Icing" vs. "Frosting"
Technically, "frosting" and "icing" are the same thing -- a combination of sugar, fat and, sometimes, decorative accents. Food writers, especially those in Europe, often interchange the terms. This owes to the fact that, while Americans tend to use the word "frosting" to describe the soft, fluffy dressing that swathes layer cakes and swirls atop cupcakes, other English-speaking countries use the word "icing" to describe the same stuff. Whatever term you choose, the sweet confectionery garnish varies in texture and application to suit whatever treat it is being applied to: thick or thin, cooked or uncooked, simple or ingredient-intensive. The key is, simply, to complement the flavor and consistency of the icing to the confection.