Beat butter on high speed with an electric mixer for about three minutes until light and airy.
Mix in powdered sugar a little bit at a time.
Add lemon zest and salt and continue to beat for another minute.
If desired, add more powdered sugar to make the buttercream frosting stiffer.
If you plan to make a large swirl of frosting, this recipe will only cover about 8 cupcakes. Double as needed.
The frosting will cover one 8" cake.
If you want to pipe it on cupcakes, the easiest way to achieve the perfect cupcake swirl is to use a 1M tip.
While there are many frosting recipes that you can mix by hand, the best buttercream frostings require lots of air to be incorporated into the butter. To really get this job done right, I highly recommend that you use an electric mixer. A hand mixer or a stand mixer will both work well.
You will need to stop and scrape down the side of the bowl every minute or so while you are mixing as butter will stick to the edges of the bowl. The easiest way to do this is with a silicone spatula. One way to avoid scraping down the bowl is to use a mixer that has a scraping paddle. Many mixers now come with scraping paddles, but you may be able to purchase one if yours doesn't (e.g. the beater blade).
Sift the powdered sugar if needed. Take a look at the actual sugar to see if it has clumps (some brands are clumpier than others). Clumps of powdered sugar will not get integrated into the frosting and may end up getting stuck in your piping tip. Measure before sifting.
The frosting lasts for up to a week at room temperature or months in the freezer. You will need to bring it to room temperature and mix it again for a minute or two before you start piping.