In a large bowl, put egg whites and sugar and whisk well. Put the bowl over a pot which has water simmering at low heat. Whisk constantly until very hot (140F or 60C).

Set it on a stand mixer and whip until hard peaks form, about 7-10 minutes. The meringue becomes shiny and cool.

Change the attachment to a paddle, and start adding butter a couple of tablespoons at a time. Scrape the sides and bottom every so often and keep adding butter until all is incorporated.

Add vanilla and mix well.