Grease a cookie sheet with butter and sift flour onto the pan, dusting off any excess.
Preheat your oven to 350°F (175°C).
Sift the all-purpose flour and baking powder in a medium bowl and set aside.
In a large bowl, beat the 4 egg whites and cream of tartar with an electric mixer until soft peaks form. Gradually add 1/2 cup of sugar and beat until stiff peaks form.
In another large bowl, beat 2 egg yolks, vanilla extract, salt, and 1/4 cup of sugar until thick and pale in color.
Beat in the sourdough discard into the egg yolk mixture.
Gently fold the egg white mixture into the egg yolk mixture until combined.
Fold the egg mixture into the flour mixture.
Pipe 3-inch (7.5 cm) long fingers onto a baking sheet lined with parchment paper.
Bake for 6-8 minutes or until the ladyfingers are lightly golden and firm.
Remove from the oven and let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Enjoy your freshly baked Sourdough Ladyfingers!
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Notes
Use a mature sourdough starter for the best flavor and rise.
If your starter is not at room temperature, take it out of the fridge and let it come to room temperature before using it in the recipe.
Be gentle when mixing the dough and avoid overmixing, as this can make the cookies tough.
Use a piping bag with a small round tip to pipe the dough onto the baking sheet for uniform cookies.
Let the cookies cool completely before storing them in an airtight container to prevent them from getting soft.
Experiment with different flavorings such as lemon zest or cinnamon to customize the flavor of your cookies.
Don't be afraid to adjust the recipe to your liking by adding more or less sugar or flour depending on your taste preferences.