Maple Meringue Buttercream

Preparation Beat egg whites with cream of tartar and salt in a large bowl with an electric mixer at medium speed until they just hold soft peaks. Add maple sugar 1 teaspoon at a time, beating, and continue to beat until whites just hold stiff peaks. Boil syrup in a small heavy saucepan over moderate heat, undisturbed, until it reaches soft-ball stage (registering 238 to 242°F on thermometer), about 3 to 7 minutes. Immediately remove from heat and slowly pour hot syrup in a slow stream down side of bowl into egg whites, beating constantly at high speed. Beat meringue, scraping down side of bowl occasionally with a rubber spatula, until meringue is cool to the touch, about 6 minutes. (It's important that meringue be fully cooled before proceeding.) With mixer at medium speed, add butter 1 tablespoon at a time, beating well after each addition. (If buttercream looks soupy after some butter is added, meringue is too warm: Chill bottom of bowl in a larger bowl of ice and cold water for a few seconds before continuing to beat in remaining butter.) Continue beating until buttercream is smooth. (Mixture may look curdled before all butter is added but will come together before beating is finished.) Cooks' notes:Buttercream can be made 1 week ahead and chilled, covered, or 1 month ahead and frozen. Bring to room temperature (do not use a microwave) and beat with an electric mixer before using. The egg whites in this recipe will not be fully cooked, which may be of concern if there is a problem with salmonella in your area. You may want to use either pasteurized egg whites in the carton or reconstituted powdered egg whites.