01 - In a large, clean mixing bowl, beat the egg whites with an electric mixer on medium speed until foamy.
02 - Incorporate the cream of tartar and continue beating until soft peaks form.
03 - Gradually add the powdered sugar one cup at a time, beating on low speed until combined, then beat on high until the mixture is thick, glossy, and holds stiff peaks, about 5 to 7 minutes.
04 - Fold in vanilla extract if desired.
05 - Use immediately for assembly and decoration or cover with a damp cloth to prevent drying if not using right away.