Note 1: These can be made a day or two ahead of time and stored in an airtight container until ready to use.
Note 2: Make sure your mixing bowl and the whisk attachments for your mixer are spotlessly clean to start. Any residual grease can make it harder to whip up the aquafaba to the correct texture.
Preheat oven to 350°F. Line 2 cookie sheets with parchment or silicone baking mats.
Add the aquafaba, cream of tartar, and about 2 tablespoons granulated sugar to the mixing bowl. Using an electric hand mixer, beat on high speed for a minute or two until the mixture is pale and very foamy.
1/2 cup aquafaba, 1/4 teaspoon cream of tartar, 1/2 cup granulated sugar
Continue beating on high speed, adding the remaining granulated sugar in gradually. Beat until the mixture reaches firm peaks. The mixture should feel very thick at this point and it should retain the ribbon patterns made by the electric mixer. If the patterns collapse or "melt" away after a few seconds, keep beating.This process usually takes about 5-8 full minutes on high speed with my hand mixer, but it can take longer depending on the aquafaba you are using. Add in the yogurt and oil. Beat for just a few seconds to incorporate.
3 tablespoons plain unsweetened vegan yogurt, 1 tablespoon vegetable oil
Sift in the flour, baking powder, cornstarch, and salt. Gently fold the dry ingredients into the whipped aquafaba mixture until no streaks of dry flour are visible.
1 ½ cups all-purpose flour, 2 tablespoons cornstarch, 1 teaspoon baking powder, 1/4 teaspoon fine salt
Allow the mixture to sit for about 5 minutes. Then transfer the mixture to a large piping bag with 1/2" piping tip. Pipe out the ladyfingers in lines, about 4" long. Leave about 2" of space between them to expand while baking. I divide the batter between 2 cookie sheets and usually get 24-28 ladyfingers.
Give one tray of ladyfingers a generous dusting of powdered sugar, then bake in preheated oven for 15 minutes, or until just slightly golden brown on the bottoms.Repeat with the second cookie sheet. (Wait to dust it with powdered sugar right before it goes in the oven, or the sugar will start to dissolve while it sits.) Bake for 15 minutes. powdered sugar
Let the ladyfingers cool on the cookie sheets before removing them.