In the heatproof bowl of a stand mixer, heat egg whites, sugar, and salt over a saucepan of simmering water, whisking constantly, until sugar dissolves and an instant- read thermometer registers 160°F (70°C).

Return bowl to stand mixer. Using the whisk attachment, beat at high speed until white and fluffy, 8 to 10 minutes. With mixer on medium speed, add butter, one cube at a time, beating until combined after each addition. (Mixture may look curdled; keep beating.) Add vanilla, and beat at high speed until thick, smooth, and well combined, 2 to 3 minutes. Use immediately.