In the heatproof bowl of a stand mixer, combine egg whites, sugar, and salt. Place bowl over a saucepan of simmering water, and cook, whisking constantly, until sugar dissolves and an instant-read thermometer registers 160°F (70°C).
Carefully return bowl to stand mixer. Using the whisk attachment, beat at high speed until white and fluffy, 8 to 10 minutes.
Add butter to mixture, one cube at a time, beating until combined after each addition. (Mixture may look curdled, but just keep mixing.) Beat at high speed for 10 minutes.
Add vanilla bean paste to mixture, and beat at high speed for 2 to 3 minutes. Switch to the paddle attachment, and beat at low speed for 1 minute to remove any air.
Recipe by Bake from Scratch at https://bakefromscratch.com/vanilla-bean-sheet-cake/