Add the egg whites and ⅓ of the sugar to the bowl of a stand mixer. Using the whisk attachment, beat on medium-high speed until soft peaks form.
In a medium saucepan, combine the remaining sugar with ⅓ cup of water. Heat over medium heat without stirring until it reaches 240°F on a candy thermometer.
With the mixer running on medium-high speed, slowly pour the hot syrup down the side of the bowl between the whisk and the bowl. Continue whipping until the bowl and mixture are cool, about 10–15 minutes.
Once the mixture is thick and holds peaks, mix in the salt.
While the mixer is running, add the butter a few pieces at a time. The mixture will look thin at first but will thicken as all the butter is incorporated.
Add the vanilla extract and beat on high if the buttercream still seems too soft.
Use immediately or store in an airtight container in the refrigerator.