Preheat the oven to 350º F (180º C) and line two baking sheets with parchment paper.
In a stand mixer, or using an electric mixer, whip 2 egg yolks and 1/3 cup of white sugar until increased in volume, pale and fluffy.
In a separate bowl, whip the 3 egg whites to a soft peak and then add the other 1/3 cup of white sugar until you get a stiff-peak meringue. Fold the egg yolks into the egg whites until just incorporated. (Make sure you don't knock out to much air during mixing!)
Sift the gluten free flour, cornstarch, and baking powder into the egg mixture. Fold everything together until no flour clumps remain, but make sure not to lose too much air during mixing.
Transfer the cookie batter into a piping bag with a medium round nozzle (the diameter of the nozzle will determine the thickness of your cookies, so choose appropriately depending on what size of cookies you want), and pipe about 4 inch (10 cm) long lines of batter onto the lines baking sheets. (Make sure to leave about 3/4 inch (2 cm) space between the cookies.)
Mix together the powdered and granulated sugars and, using a sieve, generously dust the piped ladyfinger cookies.
Bake in your preheated oven for 15 minutes, then reduce the oven temperature to 300º F (150º C) and bake for another 12 – 15 minutes. Remove from oven and cool fully before using.