Step 1

Put sugar in the bowl of a food processor, and scrape in vanilla seeds. Process until combined. Sift into a bowl, and discard any large pieces of vanilla.

Step 2

Put egg whites, vanilla-sugar mixture, and salt in the heatproof bowl of an electric mixer. Set over a pan of simmering water. Whisk constantly until sugar has dissolved and mixture registers 160 degrees on an instant-read thermometer.

Step 3

Return bowl to mixer; fit mixer with the whisk attachment. Beat on medium-high speed until stiff, glossy peaks form, and mixture is cool, about 10 minutes.

Step 4

Switch to the paddle attachment. With mixer on medium speed, add butter, 1 or 2 tablespoons at a time, mixing well after each addition. Mix, scraping down sides of bowl as needed, until the mixture is smooth. Mix in vanilla extract. Use immediately, or cover and refrigerate. Frosting can be refrigerated in an airtight container for up to three days. Before using, bring to room temperature; beat with the paddle attachment on medium-low speed until smooth.