In a medium mixing bowl, use a whisk to combine the confectioners' sugar, dried egg whites, cornstarch, and cream of tartar.
Using your flour sifter, sift the ingredients into the other medium bowl and then sift it back into the original. Repeat the process a total of four times for your meringue powder. Sifting the meringue powder is important as it provides uniform ingredient distribution.
Store your meringue powder in an airtight container in the fridge for up to a month. If desired, you can add a ceramic desiccant disk to prolong the freshness. Before using, sift your meringue powder one more time.