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A frosting knife is a kitchen tool designed to help apply frosting to a cake or to spread any easily malleable substance across food. The handle of a frosting knife is usually symmetrical, as is the rest of the knife, so it can be easily manipulated from any angle. The blade itself is often straight on each of the longer sides and curved at the top so there are no hard edges in the profile. The sides of the blade are not sharp, and the metal is made to be firm, but also to have some slight give so it does not destroy delicate cakes when being used. One variation of a standard frosting knife has the blade offset below the level of the handle, creating space where the fingers can grip the knife without falling below the frosting line when being used.
Nearly any material can be used to make a frosting knife. Wood, plastic and even metal are common for the handle. Steel, aluminum or plastic can be used to make the blade. Most knives are designed to be lightweight so the knife does not interfere with the application of frosting to a cake. A few knives are serrated on one side to cut the cake and also to scrape patterns into the frosting, if needed.
A frosting knife is most effective if it is used after a cake knife. A cake knife is similar in some ways to a frosting knife, except it has a longer, thinner blade and a serrated edge designed to cut through cake. Many cakes come out of an oven with uneven surfaces, so the cake knife can be used to cut a clean and flat surface on the top and sides of a cake. With a flat surface on the cake, the frosting knife can easily spread room-temperature frosting across the surface to provide a flat, professional-looking layer.
There are other uses for a frosting knife aside from decorating cakes. It can be used to cut and spread ice cream. When a savory meal is being made, the knife can be used to spread butter over a large surface such as a loaf of bread. The knife also can be used to spread tomato sauce on a lasagna or pesto on bread. The design makes it possible to use the knife to spread almost any filling or sauce over a flat surface.