Step 1

Combine sugar and water in a medium saucepan. Heat over low heat, stirring, until sugar has dissolved and the syrup is clear. Increase heat to medium-high and allow syrup to come to a boil.

Step 2

Place yolks in the bowl of a stand-mixer fitted with the whisk (or, if using a handheld mixer, in a medium mixing bowl) and beat until thick and foamy.

Step 3

Cook syrup until it registers 235°F, then immediately remove from heat. With the mixer running, slowly drizzle hot syrup into the bowl with the yolks: don't pour the syrup onto the whisk, or the syrup may splatter against the sides of the bowl; instead, aim for a spot close to the whisk.

Step 4

Once all the syrup has been added, keep mixing until the bottom of the bowl feels cool to the touch and the yolk mixture has cooled to room temperature.

Step 5

Add butter one cube at a time, beating well after each addition. Beat in vanilla and salt. Continue mixing until the buttercream looks smooth and creamy, about 5 minutes. (If the buttercream separates, keep mixing: it will come together eventually.)