In the heatproof bowl of a stand mixer, combine the egg whites and granulated sugar. Place the bowl over a saucepan of simmering water to create a double boiler, ensuring the water doesn't touch the bottom of the bowl. Whisk constantly until the sugar is fully dissolved and the mixture reaches 160°F (71°C).

Transfer the bowl to your stand mixer fitted with the whisk attachment. Beat on medium-high speed for 10-15 minutes, until the meringue is stiff, glossy, and the outside of the bowl has cooled to room temperature.

Switch to the paddle attachment. With the mixer on low speed, add the softened butter one tablespoon at a time, waiting for each to be incorporated before adding the next.

Once all butter is added, the mixture may look curdled. Increase the speed to medium-high and whip for 3-5 minutes until it becomes thick and silky smooth. Mix in the vanilla extract and salt on low speed until combined.