In the bowl of a stand mixer fitted with the paddle attachment, beat the egg yolks on high speed until very pale in color and slightly thickened.
In a small saucepan, combine the sugar and a ½ (120g) water and place over medium heat, stirring on occasion.
Once the mixture starts to boil, stop stirring and boil the mixture until the sugar is dissolved and reaches 238F on a candy thermometer. Remove from the heat.
With the mixture on low speed, slowly stream in the hot sugar syrup. Be sure to do this slowly and on low speed so the hot sugar doesn't splash up.
Turn the mixer up to medium and beat until the mixture is room temperature and the bowl is completely cool.
Add the butter, a few pieces at a time. Add in the salt and vanilla.
Beat until the buttercream is completely smooth and silky (about 5 minutes).