Add the 1 ½ cup granulated sugar and ½ cup water to a small saucepan or skillet and heat over medium-low heat until boiling. Reduce the heat and cook the sugar while preparing the eggs.
Add 8 large egg yolks to the bowl of a stand mixer and whip medium-high with the whisk attachment until the yolks are thick and foamy.
When the sugar reaches 240 degrees Fahrenheit, remove the pan from the heat and turn the mixer on low speed. Slowly and carefully drizzle the sugar syrup into the egg yolks as they whip.
Once all the sugar has been incorporated into the eggs, continue to whip at medium speed until the bowl feels cool to the touch.
Add 1 ½ cups unsalted butter, one tablespoon at a time, waiting for each tablespoon to be absorbed into the eggs before adding the next.
Add 2 teaspoons vanilla extract, and 1 pinch salt. Continue to whip on medium speed until the buttercream is smooth and glossy.