A well-decorated cake is a beautiful thing, but in some climates keeping it beautiful can present a challenge. Butter, the main ingredient in traditional buttercream frostings, becomes soft at ordinary room temperatures and melts when the mercury rises past the mid-80s Fahrenheit. Shortening-based frosting is sturdier, but not nearly as tasty. Thickening and stabilizing your frosting with gelatin is a better alternative that improves its hot-weather performance without compromising on flavor.
Combine 1 part unflavored gelatin powder with 3 parts cold water in a small bowl or measuring cup. Generally, allow 3/4 teaspoon to 1 teaspoon of gelatin for each pound of butter in your frosting recipe. Use the larger measure for especially light and airy frostings, such as Swiss or Italian buttercream. Allow the water-gelatin mixture to "bloom" for five minutes, until the gelatin granules have become thoroughly moistened and swollen.
Melt the gelatin mixture over a cup of boiling water, stirring it periodically until it's completely dissolved. Alternatively, melt the gelatin in your microwave in 20-second increments at half-power, stirring after every "zap" until it's completely dissolved.
Prepare the frosting, using your favorite recipe. If the frosting was made ahead of time, whip it briefly in your mixer to ensure that its texture is light, smooth and malleable.
With your mixer running, pour the melted gelatin into your frosting in a very thin stream. Whip the frosting until the gelatin has been completely dispersed and is no longer visible.
Frost your cake immediately, as soon as the gelatin has been incorporated. Once it sets, you won't be able to create a smooth surface or clean-edged piped decorations any longer.
Gelatin begins to melt at temperatures approximately 10 degrees higher than butter, in the mid-90s F. However, food-science writer Harold McGee notes that sugar -- plentiful in frosting -- strengthens gelatin, and in practice it's usable in all but the very hottest climates. A seaweed-derived gelatin substitute, agar-agar, sets to a stiffer texture but won't melt through atmospheric heat. If you live in a truly torrid climate, it's a better option than conventional gelatin.
Gelatin is most useful with meringue-based Swiss or Italian buttercreams, which are especially delicate and prone to melting because of their airy texture. Ordinary buttercreams and even cream cheese icing are sturdier, but can require gelatin's assistance on the hottest of days. Gelatin can even be added to make delicate whipped cream usable as frosting, though even with that assistance it's not suitable for warm-weather cake decorating.