In large bowl, whip meringue powder and 1/2 cup cold water with electric mixer on high speed until stiff peaks form, about 4 minutes.
In saucepan, stir together sugar, corn syrup and remaining 1/2 cup water. Bring to boil; cool 1-2 minutes. Slowly add syrup to meringue mixture, streaming down side of mixing bowl. Whip on high 4 minutes. Add vanilla extract and whip until well combined. Use immediately to ice cakes or cupcakes.