1. Add the egg whites and cream of tartar to the bowl of a stand mixer. Beat on medium speed until foamy, then continue beating until soft peaks begin to form.

2. In a small heavy-bottomed saucepan, combine the granulated sugar and water. Cook over medium heat until the sugar dissolves, then continue cooking without stirring until the syrup reaches 240°F.

3. With the mixer running on medium speed, carefully pour the hot syrup into the egg whites in a thin steady stream along the side of the bowl.

4. Increase the mixer speed to medium-high and beat until the meringue is glossy and the bowl has cooled to room temperature.

5. Add the softened butter one tablespoon at a time, mixing well after each addition. Continue beating even if the mixture looks curdled or loose.

6. Add the vanilla extract and salt, then beat until the buttercream becomes smooth, fluffy, and silky.

7. Use immediately to frost cakes or cupcakes, or store and rewhip before using.