In the heatproof bowl of a stand mixer, combine sugar, egg whites, and salt. Place bowl over a saucepan of simmering water, and cook, whisking frequently, until sugar dissolves and an instant-read thermometer registers 130°F (54°C).

Carefully return bowl to stand mixer. Using the whisk attachment, beat at high speed until fluffy and bowl is cool to the touch, 6 to 8 minutes. Add butter, 1 tablespoon (14 grams) at a time, beating until combined after each addition. (If buttercream breaks, beat for 2 to 3 minutes, and emulsion will come back together.) (See Kitchen Tips.) Reduce mixer speed to medium, and beat until smooth and shiny. Add melted white chocolate, and beat just until combined.