Combine sugar, cream of tartar, and water in a heavy saucepan. Cook over medium heat, stirring frequently, until mixture comes to a boil and the sugar is dissolved. Continue cooking, stirring frequently, until the mixture reaches soft ball stage (240°).
Beat egg whites (at room temperature) until foamy. While beating at medium speed of electric mixer, slowly pour hot syrup in a thin stream over egg whites. Turn mixer to high speed and continue beating until stiff peaks form. Add flavorings; beat until blended. Stir in remaining ingredients. Reserve 1/4 cup prepared filling for garnish.