Sift the flour, cornstarch, and salt in a bowl. In a larger bowl, beat 1/2 cup confectioners' sugar with butter using an electric mixer until the mixture is creamy and free of lumps. Beat in the lemon extract, then mix in the flour mixture to make a smooth dough. Chill the dough for 1 hour.
Preheat oven to 300 degrees F (150 degrees C). Mix 1 cup of confectioners' sugar with the lemonade drink mix in a plastic zipper bag. Close the zipper, and shake the bag to blend.
Scoop up about 1 1/2 teaspoon of dough per cookie, and roll into balls. Place the cookies onto ungreased baking sheets.
Bake in the preheated oven until the cookies are set, about 20 minutes. Remove from oven, and let cool on sheets for 2 minutes.
Drop the warm cookies into the bag of lemon sugar, about 5 or 6 at a time, and shake to coat with the mixture. Shake off excess sugar, and allow the cookies to finish cooling on wire racks. When cool, shake them in the lemon sugar one more time.
