Mix granulated sugar, cream of tartar, and ¼ cup of water. Boil in a saucepan over medium-high heat. If you have a candy thermometer (Amazon affiliate link) -I do recommend using one- boil until it reaches 112 °C [235 °F]. If you do not have a thermometer, boil until it reaches a syrup consistency (It will be thick but still a light color, see notes)
2. Whisking egg whites
In the meantime, using the stand mixer wire attachment, whisk the egg whites for 30 seconds at low speed. Increase speed to medium and whisk until they become white and has doubled and formed peaks (about 3 mins).
3. Adding syrup
Slowly, and in a thin stream, pour the syrup into the bowl while whisking over high speed until it forms very firm peaks (about 5 mins).
4. Finishing
Add the vanilla extract, and once incorporated, the powdered sugar, whisk until it's combined and the meringue is at room temperature. The meringue should have firm peaks, and some of the meringue has "climbed" into the center of the wire whisk.
5. Storing
It can last a few hours well covered and refrigerated. If you notice that it starts to form a crust, add a few drops of lime juice, and incorporate with a spatula.
Video
Notes
To make Suspiro your utensils must be very clean. Any trace of grease will make it impossible to form peaks. I suggest you wash them with warm soapy water and dry carefully beforehand.When separating yolks from white, make sure there's absolutely no trace of yolk in the egg whites, otherwise, it will not form and keep stiff peaks.Humidity in the air and room temperature will affect the results when preparing 'suspiro'. Hot, dry days yield better Suspiros.Cream of tartar is an acidic white powder used in baking to activate acid/base reactions without altering the flavors, in this case, it improves the texture and "airiness" of the Suspiro. It can be found in the baking aisle of your local supermarkets. It's very cheap and lasts for years.Adding dark vanilla extract instead of clear will produce a creamy-colored suspiro (as opposed to its natural bright white color). This may be something to work into your design.Please use a stainless steel or glass mixing bowl. Plastic may not be resistant to the high temperature of the syrup, and may be difficult to clean from all traces of fat.