In a bowl combine flour, cornstarch, baking powder and salt and whisk for 30 seconds, set aside. In the bowl of an electric stand mixer, whip together butter, shortening, sugar and lemon zest on medium speed until very pale and fluffy, about 4 minutes, occasionally scrapping down the sides of the bowl. Add in egg and mix until combine, then add egg white, lemon extract and vanilla extract and mix until combine. With mixer running, slowly add in dry ingredients and mix just until combine. Transfer dough to an airtight container and refrigerate 2 hours. Preheat oven to 375 during the last 10 minutes of refrigeration.
Scoop dough out an even 1/4 cup at a time and roll into a ball. Place dough ball on a lightly floured surface, sprinkle top lightly with flour, then using something flat and smooth (such as the storage container lid), press and evenly flatten dough to 1/2" thick (so you should have about a 2 2/3" - 2 3/4 " circular cookie). Transfer flattened dough to a Silpat lined cookie sheet (fitting 6 cookies per cookie sheet) and repeat process with remaining dough. Bake in preheated oven 9 - 11 minutes. Allow to cool 5 minutes on baking sheet before transferring to a wire rack to cool. Cool completely then frost with Lemon Frosting.
For the lemon frosting:
Combine all ingredients in a large mixing bowl, using an electric hand mixer, whip until frosting is smooth and fluffy. Mix in food coloring if desired or sprinkle cookies with yellow sprinkles.
Recipe Source: Cooking Classy