Prepare a baking tray with a baking mat or parchment paper. I like to use a Perforated Air mat.
Mix sugar with cream of tartar in the cup. This will help to avoid any clumps.
In the mixing bowl of a stand mixer with whisk attachment, add egg whites. Start whipping on low speed.
When egg whites become foamy, start adding white granulated sugar and cream of tartar.
Change speed to medium-high and whip until stiff peaks. Meringue should be glossy and thick.
Sift powdered sugar into meringue and whip again for about 20-30 seconds.
Transfer the meringue into a prepared pipping bag with a pipping tip. I would recommend using a coupler, this way, you can change piping tips and pipe different shapes of the meringue cookies.
Bake Meringue Cookies in a preheated oven at 220oF (100oC) for 60-90 minutes. Do not open the oven during baking.
Cool Meringue Cookies to room temperature. After that, transfer the cookies to an airtight container. Store at room temperature for 1-2 weeks. Do not refrigerate!
Notes
Use room-temperature egg whites.Do not open the oven door during baking.