
Combine sugar and water in medium saucepan. Heat over low heat while stirring until sugar dissolves. Increase heat to medium- high and bring to a boil
Put egg yolks in a stand-mixer fitted with a whisk attachment and beat until thick and foamy.
Cook the sugar and water syrup until it reaches 240 degrees F. Immediately remove from heat. With mixer running, SLOWLY drizzle hot syrup into bowl with yolks.
Continue mixing until the bottom of the bowl is cool to the touch and the yolk mixture has cooled to room temperature.
Add in butter one cube at a time allowing each piece to incorporate before adding the next. Add vanilla and salt. Continue mixing until buttercream is smooth and creamy. (About 5-6 minutes.)