This is absolutely the best icing I have ever made.
1 teaspoon lemon juice, or as needed
4 cups white sugar
2 cups egg whites
5 cups unsalted butter, cut into cubes
2 tablespoons pure vanilla extract
¼ teaspoon salt
Wipe the bowl of an electric stand mixer and the whisk attachment with lemon juice to remove any trace of grease. Add sugar and egg whites to the bowl.
Whisk sugar and egg whites together in the bowl of the stand mixer over simmering water, whisking constantly but gently, until temperature reaches 140 degrees F (60 degrees C), about 10 minutes. Return bowl to the stand mixer.
Whisk sugar-egg white mixture in the stand mixer fitted with the whisk attachment until mixture is thick, glossy, and temperature of outside of bowl is room temperature. Change attachment to the paddle attachment and continue mixing on low speed. Add butter, 1 cube at a time; mix until silky-smooth. If mixture curdles, continue mixing and it will return to smooth.
Pour vanilla extract and sprinkle salt into butter-sugar mixture; beat until fully incorporated.