The key to beautiful piping on a cake or cupcakes is having your icing at the correct consistency. If icing is too thin, it will not hold its shape well and can run. Icing that is too thick may be too hard to squeeze or may crack once dry. Achieving the perfect consistency is a simple two-step process of making your base icing and then adjusting the ingredients slightly to make your icing stiff enough for piping your decorations. The following recipe is a commonly used cake decorator's recipe for piping decorations.
Place the shortening in your stand mixer equipped with the paddle attachment. Cream the shortening on medium speed for 2 to 3 minutes until it gets light and fluffy.
Add the salt and butter and vanilla flavourings to the shortening. Mix on medium speed until the flavourings are fully incorporated into the shortening.
Place your mixer on the lowest speed and add in one cup of the icing sugar. Beat until the sugar is fully incorporated. Stop the mixer and scrape down the sides of the bowl with your spatula.
Repeat with the remaining powered sugar -- adding one cup at a time -- until all of the icing sugar is incorporated. Once all the sugar is added, place your mixer on medium speed and beat the frosting for 2 to 3 minutes more.
Test the consistency of your icing. Icing that is thin but will still hold when piped should hold a 1/4 inch. peak when you dip in a spatula. Medium consistency icing peaks at 1/2 inch and stiff icing peaks at 3/4 inch.
Add in milk or water 1 tsp at a time and beat on low speed until fully incorporated if you need to thin out your icing a little bit. Just a few small drops of liquid can completely change the consistency of your icing, so use liquid sparingly and make sure it is fully incorporated before you add more to your icing.
Add in more icing sugar -- 1/4 cup at a time -- if your icing is too thin for your desired piped decoration. Beat the icing well after each addition so that the sugar is fully incorporated and no clumps are left to clog up your piping bag.
Stiff icing is used mainly for roses and other flower petal decorations. Make your icing a little thinner for writing or for other decorations such as shell or star borders.
Be aware that factors such as heat and humidity can affect the consistency of your icing. Keep your icing covered when you are not using it to prevent it from drying out.