Bring the sugar and water to a boil in a small saucepan.
Cover with a lid or plastic wrap and let boil for 2-3 minutes. Remove the lid and cook the sugar to 240F.
While your syrup is boiling, whip the yolks and salt on high speed until very light and thick and tripled in volume.
When the syrup has reached temperature, turn down the mixer speed and slowly but steadily pour the syrup in a thin stream down the inside of the mixer bowl.
***Make sure the syrup is sliding down the inside of the bowl--this will help to cool it off a little before it reaches the eggs. It will also keep molten sugar from splattering all over the inside of your mixer instead of ending up where it belongs: in the buttercream.
Increase speed to high and whip until cool.
Whip in the butter a little at a time. Don't add it too quickly. Wait until what you've put in has been completely blended in before you add some more.
Beat in the vanilla.
Notes
You can pretty much add any extract or liqueur to taste. To make chocolate buttercream, whisk in 6 oz. melted and cooled bittersweet chocolate. For a fruit flavored buttercream, whisk in 4-6 oz. fruit puree. To make caramel buttercream, cook the sugar syrup to a medium amber caramel before adding it to the eggs.