In a heatproof bowl set over a saucepan of simmering water (double boiler), combine the egg whites and granulated sugar. Whisk constantly until the sugar has completely dissolved and the mixture reaches 160°F (71°C) on a kitchen thermometer.

Transfer the bowl to a stand mixer fitted with the whisk attachment. Whip on high speed for 10-15 minutes, until the meringue is glossy, thick, and forms stiff peaks. The outside of the bowl must be completely cool to the touch.

Switch to the paddle attachment. With the mixer on medium-low speed, add the softened butter one tablespoon at a time, allowing each to incorporate before adding the next.

Continue mixing after all butter is added. The mixture may look curdled or soupy; this is normal. Keep mixing for another 5-10 minutes until it becomes thick, smooth, and silky.

Add the vanilla extract and salt. Mix on low speed for another 2 minutes to make it perfectly smooth.