If you just want to do a thin layer of frosting around the outside, the recipe is the correct amount. However if you want to decorate with the frosting you'll want to 1.5x it.
In the bowl of a stand mixer add in the egg whites, sugar, and salt. Whisk to combine.
Place a medium sauce pot half way full with water onto the stove. Bring the water to a simmer.
Set the mixing bowl with the egg whites and sugar over the bowl of simmering water and turn the heat to low. The water shouldn’t touch the bottom of the mixing bowl.
Whisk the egg whites and sugar constantly until the sugar has fully dissolved into the egg whites. To check and see if the sugar has dissolved, pinch and rub some of the mixture in between your fingers and feel if you still feel any of the sugar granules, if not you’re good to go. If you do just keep cooking a bit longer.
Once the sugar has dissolved, turn off the heat and remove the bowl from the water. Leave it to cool off for about 5-10 minutes until the bottom of the bowl is just warm to the touch.
Using your mixer, whip the egg white mixture first on medium and then on high speed until fluffy, shiny and stiff.
The bottom of the bowl should have mostly cooled off at this point.
Once coolish to the touch, turn the mixer to medium high speed. Add in 1″ blocks of butter at a time whipping until all of the butter has mixed in. It will deflate a little bit at the beginning, that’s ok!
If your buttercream starts to look split, the butter maybe too cold so just let it run for a bit longer. If it looks soupy, it just needs to chill so either put an ice pack or ice bath under the bowl. Or place the bowl in the fridge for 5-10 minutes.
When almost all of the butter has been added, the mixture is going to look funny, just keep whipping and it will smooth out. Stop the mixer and add in the lemon zest, juice, and vanilla. Then mix again until smooth. Add more juice to taste!