Ingredients
7 extra-large egg whites
2 1/2 cups sugar
2 teaspoons cream of tartar
1 cup (2 sticks) unsalted butter, cut into small pieces and chilled
1 teaspoon pure vanilla extract
2 vanilla beans, halved lengthwise
Preparation
In a large heatproof bowl set over (but not touching) simmering water, whisk together egg whites and sugar until sugar is melted and mixture reaches 140 degrees on a candy thermometer, 7 to 10 minutes.
Transfer mixture to the bowl of an electric mixture fitted with the whisk attachment; beat on high until stiff peaks form, 12 to 15 minutes. Add cream of tartar and beat for one minute more.
Reduce speed to medium and add butter, a few pieces at a time, mixing well after each addition. Add vanilla extract and scrape vanilla bean seeds into mixture; beat until well combined, about 2 minutes. Transfer to refrigerator; chill for 15 minutes before using.
