The buttercream frosting that I prefer to cover and decorate my cakes with is called Italian Meringue Butter Cream (a.k.a. IMBC). I love this buttercream because the taste is so creamy and smooth and relatively easy to work with.
There are only 3 basic ingredients needed for this buttercream frosting: egg whites, sugar and butter. The process involves making a meringue by boiling the sugar into the firm ball stage and pouring it into the the egg whites to cook it. Butter is added once the mixture has cooled down.
I prefer to use pasteurized egg whites that comes in a carton. It is very convenient to just pour and measure it in a cup rather than break and separating the yolks from the white.
The butter should be soft but on the cool side. If the butter is too soft and warm the resulting buttercream will be too thin and will have a hard time holding up its shape. When this happens, place the bowl in the fridge to cool it down. You can also set the bowl on top of ice cold water. Mix it often with a spatula to avoid hardened clumps.
This buttercream frosting sets hard when refrigerated due to its butter content. It is best to serve it at room temperature; take the cake out of the fridge one hour before serving it. Serving the cake cold or at room temperature also goes by personal preference. I have a friend who likes eating the frosted cake from the freezer because the frozen buttercream "tastes like ice cream".
You can freeze unused buttercream frosting in an airtight container for up to 6 months. To use it, bring the buttercream to room temperature and using the stand mixer, beat it with the spade beater. You can also plan ahead by putting the frozen buttercream frosting in the fridge the night before, this will shorten the waiting time. Sometimes it will look crumbly or a curdled mess but don't get discouraged. It is simply a tad bit cold and you can remedy this by either placing it in the microwave for a few seconds or putting the bowl on top of warm water. Always stir the rubber spatula to make it smooth.
I have tried different IMBC recipes and I found that Kate's Buttercream from the book The Whimsical Bakehouse: Fun-to-Make Cakes That Taste as Good as They Look! will give the best capacity for my stand mixer. It yields about 8 cups of frosting, generous enough to fill and frost up to 2 - 10'' round cakes. You can flavor it up by adding fruit purees and flavored liquor. An did I also mention that it tastes incredibly good?
Adapted from The Whimsical Bakehouse: Fun-to-Make Cakes That Taste as Good as They Look by Kaye Hansen.