In the bowl of a stand mixer set over a pan of simmering water, combine egg whites, sugar, vanilla, and reserved vanilla bean seeds. Whisk constantly until a candy thermometer reads 140°.
Transfer bowl to stand mixer, and beat mixture with whisk attachment at high speed until cooled and tripled in volume, 10 to 12 minutes.
Reduce mixer speed to low. Add butter, a little at a time, beating well after each addition. Remove whisk attachment, and replace with paddle attachment. Beat mixture at medium-high speed until light and fluffy, about 4 minutes. Use immediately.