Pour the water in a small saucepan. Top with the granulated sugar, reserving a ¼ cup. Don’t stir. Just put it on the burner on medium heat. Clip on your candy thermometer.
Place the egg whites in the bowl of an electric mixer fitted with the whisk attachment. Add a pinch of cream of tartar. Start the mixer on medium-low to begin frothing the whites.
When the sugar begins to boil, increase the speed of the mixer to medium-high. When the whites are at soft peaks, gradually add the reserved sugar. Continue beating to stiff (not dry) peaks.
When the sugar reaches 112 ºC or 233 ºF (you can go up to 121 ºC or 250 ºF), remove the pan from the burner. Turn off the heat, unclip the thermometer and then slowly pour the hot sugar in a fine stream down the side of the bowl, being sure not to hit the beater (or it will splash!).
When all the sugar is added, stop the mixer and scrape down the sides of the bowl with a spatula. Increase the speed to high and beat til the mixture has just about cooled (this takes a good 5–10 mins for such a large batch).
When the meringue has cooled, start adding the butter, a tablespoon at a time, while the mixer is running on medium.
When all the butter has been added, increase the mixer to high to beat until the buttercream forms and is smooth.
Beat in the vanilla and the salt.
Notes
Calories calculated per tablespoon of buttercream made according to this Italian meringue buttercream recipe