Mix together the egg whites, sugar, and salt in the bowl of your stand mixer. A metal bowl will work best for this as meringue is easier to whip up in metal rather than plastic.
Fill a pot with about 2 inches of water and bring it to a simmer.
Place the bowl of your stand mixer on top of the pot of water to create a bain-marie. The water should not touch the bottom of the bowl.
Whisk the egg whites every 30 seconds or so to distribute the heat evenly. The mixture is ready once it reaches 110ºF (43ºC), or when you can no longer feel any grains of sugar in the egg whites.
Remove the bowl from the heat and transfer it to your stand mixer. For this next step you will be using the whisk attachment.
Whip the ingredients on high speed for 10-15 minutes, or until you reach stiff, glossy peaks.
Pour your meringue out into a shallow dish and pop it into the fridge for about 10 minutes to cool the meringue. You can also put your entire bowl of meringue into the fridge, but it will take longer to cool. If you don’t cool the meringue it will melt your butter. Then you’ll have buttercream soup.
Once your meringue is cooled, put it back in your stand mixer bowl with the whisk attachment.
Turn the mixer on medium/low speed (setting 2 on my Bosch Mixer, or setting 4 on a KitchenAid) and add in your room temperature butter in chunks until it’s all combined.
Add in your vanilla and salt. If you’re flavoring your buttercream or making it into chocolate buttercream, now is the time to add those flavorings.
Bump up the speed to medium/high (setting 3 on my Bosch Mixer, or setting 8 on a KitchenAid) and whip it until your buttercream is WHITE and fluffy. It should not taste buttery. Pro-Tip: You might have a slight yellowish tinge depending on the brand of butter you use. You can counteract a yellow tint by adding in a drop or two of violet food color gel.
Switch to a paddle attachment and mix it on low for 15-20 minutes to make the buttercream very smooth and to remove air bubbles. This isn’t required, but if you want really creamy frosting, you do not want to skip it!