You may have noticed that most of my buttercream recipes don't call for butter. This is for a few reasons. First, butter needs to be refrigerated. If it is on your cupcakes, they too need to be refrigerated. I don't know about you, but my fridge is prime real estate. I don't have room for cupcakes in there! Secondly, vegetable shortening (which I use instead of butter) makes the icing the perfect consistency for decorating and it is easy to make it thicker or thinner depending on your preference. Thirdly, shortening is white and will keep your icing true to any color you add to it. If you use butter you will have a cream-colored icing, not white.
The main buttercream recipe I use is extremely forgiving. You can substitute different flavorings and liquids. You can add color or chopped nuts. Just play around with it! Here is the recipe for Vanilla Buttercream:
½ cup vegetable shortening
½ teaspoon salt
4 teaspoons vanilla extract
6 cups powdered sugar
½ cup milk
In a large bowl, mix together shortening, salt, vanilla, and 2 cups of the powdered sugar. Add in remaining sugar, 2 cups at a time and alternate with milk. Add more milk to thin and more sugar to thicken the icing, if needed. Beat on high speed for 10 minutes until light and fluffy.
When you make buttercream, pay attention to heat and humidity. If the air is very dry, you probably won't need all of the liquid. If it is very humid, be prepared to use more powdered sugar. Also, if you want the icing to be thinner, add more liquid a teaspoon at a time. For thicker icing just add more sugar. Get it the consistency you want before you add color. Gel coloring works best as it doesn't add any more liquid to the recipe and it only takes a little bit to get a vibrant hue.
If you don't want to use vegetable shortening there are other options. Simply replace all or a portion of it it with butter or cream cheese. Because these are softer you may need to adjust the amount of liquid and powdered sugar in the recipe to get it just right. You will also need to keep the icing refrigerated.
If you need a super stabilized icing for making flowers and other embellishments that doesn't require refrigeration, you will want to add meringue powder to your recipe that uses vegetable shortening. Most cans of meringue powder have a recipe on them to help you along.
I hope these tips help you out! Just remember that it isn't an exact science. Play around and have fun!