Buttercream is the utility player of the pastry world. While it can be smoothed on top of cakes, it can also help glue fondant to a cake's surface. Stiff buttercream has many uses, but if the consistency isn't just right it can be difficult to work with. To get a stiff consistency you need ingredients, temperature and technique to work together.
Stiff buttercream is used for frosting decorations that need to hold in an upright position. Buttercream flowers, petals, borders, stringwork, piping and dots need a stiff buttercream. If the buttercream is too soft decorations can droop. Stiff buttercream is very sweet; therefore it should only be used for decorations and not covering a cake.
Heat and humidity can be detrimental to buttercream. Since buttercream is made with butter and milk, heat softens its consistency -- even when made thick. Control the temperature in your work area by keeping the room below 75 degrees Fahrenheit. Since humidity adds moisture to buttercream, which prevents it from stiffening, you may need to wait for a less humid day to make your buttercream batch. Once made you can freeze it for up to two months.
There are a few variations to the traditional American buttercream. For a stiffer buttercream you can use a combination of butter and shortening, milk, confectioners' sugar and any extracts you want. The more confectioners' sugar you add, the stiffer your frosting will be. To make it move through a piping bag easier or smooth enough for intricate designs you can add a little corn syrup. Corn syrup adds elasticity, but won't water down the buttercream or make it soft. For flowers or designs that need to hold their shape upright, add 1 to 3 tablespoons of cornstarch for additional stiffness.
Mix your buttercream using the lowest speed setting on your mixer or by hand. Too many air bubbles can create an airy, soft buttercream. Since buttercream requires a lot of confectioners' sugar, it needs to rest overnight to absorb the sugar. You can store it in the refrigerator in an airtight container and bring it back to room temperature before using.