Place the egg whites and granulated sugar in a heatproof bowl. Set it over a saucepan of simmering water (double boiler) and whisk constantly. Heat until the mixture reaches at least 131°F (55°C) and the sugar is fully dissolved. Rub a bit between your fingers—there should be no grains left.

Pour the mixture into the bowl of a stand mixer fitted with the whisk attachment. Beat on high speed for 15–20 minutes, until the meringue is glossy, forms stiff peaks, and is completely cool.

Lower the speed and gradually add the softened butter, a few pieces at a time. Once all the butter is added, increase to high speed and beat for about 5 minutes. The texture may look curdled or runny at first—keep mixing until it turns smooth and creamy.

Add the vanilla extract, a pinch of salt, and the melted (but slightly cooled) white chocolate. Mix for 2–3 minutes until well combined.

Switch to the paddle attachment or mix by hand with a spatula. Beat on low speed for 2 minutes to remove air bubbles and create a smooth, silky buttercream.