Yield: approximately 3 cups

3 egg whites
¾ cup sugar
½ teaspoon vanilla extract
1/8 teaspoon cream of tartar
1 cup unsalted butter, softened
2 cups natural unsweetened cocoa powder

1. In the top of a double boiler, combine egg whites, sugar, vanilla, and cream of tartar. Set bowl over a saucepan of simmering water. Cook, whisking constantly, until egg-white mixture registers 140° on a candy thermometer. Remove from heat. Beat at medium-high speed with an electric mixer until whites are fluffy and cooled, approximately 10 minutes. Add vanilla, and continue to beat until stiff, glossy peaks form.

2. Reduce mixer speed to medium-low. Add butter, 1 tablespoon at a time, mixing well after each addition. Sift cocoa over buttercream, beating to combine. Use immediately, or refrigerate for up to 3 days.

3. Buttercream may be frozen in an airtight container for up to 1 month. Bring to room temperature, and beat on lowest speed until smooth, approximately 10 minutes. Reserve 1 cup to make Meringue Mushrooms.