Dry Ingredients: To a medium mixing bowl, add the gluten-free flour, potato starch, baking powder, and salt. Whisk together well.
1 ⅓ cup all purpose gluten-free flour, ½ cup potato starch, 2 teaspoons baking powder, ¼ teaspoon salt
Whipped Aquafaba: To a large mixing bowl, add the aquafaba and lemon juice. Whip with an electric mixer on high speed until soft peaks are formed. Then reduce the speed slightly to medium high while adding the sugar about 1 tablespoon at a time. With the addition of sugar, stiff peaks will form and the mixture will turn shiny and smooth (similar to making meringues).
½ cup aquafaba, 1 teaspoon lemon juice, ⅔ cup granulated sugar
Ladyfinger Batter: Sift the dry ingredients into the whipped aquafaba about ¼ cup at a time, folding it in by hand with a wooden spoon or spatula until mostly mixed before the next addition. Do not use an electric mixer for this step. Be careful to mix slowly and only as much as needed. When about ¾ of the dry mixture has been added, fold in the vanilla extract and melted butter. Then sift in the rest of the dry ingredients and mix only until just combined.
2 teaspoons vanilla extract, 4 tablespoons vegan butter
Shape the cookies: Preheat the oven to 400 degrees F and line two large baking sheets with a silicone liner or parchment paper. Gently add the batter to a piping bag with a ½ inch piping tip or corner cut-out. Pipe thick 4-inch logs, moving slowly so that the width of the piped ladyfinger is actually wider than the piping tip (it should push into the previously piped batter causing it to expand beyond the tip width). Leave about 2 inches between ladyfingers.
Bake: Bake for 5-8 minutes until just set but not browning. Allow to cool for 10 minutes on the pan before gently moving to a wire cooling rack using a spatula (transfer as soon as possible to a cooling rack if over-baked). Let cool completely before assembling the tiramisu, or better yet use them after a day or two when they are stale.