Grease non-stick baking sheets or line regular baking sheets with baking parchment; grease parchment well.
Preheat oven to 400°F (205°C).
Put 6 egg yolks and granulated sugar into a large bowl.
Beat until pale and creamy, 5 to 10 minutes with an electric mixer.
Beat egg whites until stiff; fold into egg yolk mixture.
Sift flour with cornstarch; fold into egg mixture.
Put mixture into a pastry bag fitted with a large plain nozzle.
Pipe 3-1/2-inch (8.5-cm) long fingers onto prepared baking sheets, leaving space between each for spreading during cooking.
Bake 7 to 10 minutes or until golden.
Cool a few seconds on baking sheets then transfer to a rack to cool completely.
Sift powdered sugar over cooled cookies.