Vanilla French Buttercream

From the Martha Stewart Show Today

Makes about 1 quart

• 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

Directions

1. 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.
2. 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.
3. 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.