Freshly made buttercream is very light and fluffy since you have just incorporated a lot of air into it. This consistency makes it easy to spread in a thin layer, giving you the advantage of needing less frosting to work with, making your dessert less rich.
However, there are times when it is more convenient to prepare the buttercream in advance and refrigerate it until you are ready to use it. If this is the case, allow the buttercream to stand at room temperature for about 1 hour; then mix it together with a rubber spatula until smooth. Or use the following method to bring it to a smooth and very spreadable consistency.