In a bowl combine cake flour, cornstarch, baking powder and salt and whisk for 30 seconds, set aside.
In the bowl of an electric stand mixer, whip together butter, vegetable shortening, granulated sugar and lemon zest on medium speed until very pale and fluffy, about 4 minutes, occasionally scraping down the sides of the bowl.
Add in egg and mix until combined, then add egg white, lemon extract and vanilla extract and mix until combined.
With mixer running, slowly add in dry ingredients and mix just until combined.
Transfer dough to an airtight container and refrigerate 2 hours. Preheat oven to 375ºF 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, press and evenly flatten dough to 1/2 inch thick.
Transfer flattened dough to a Silpat lined cookie sheet and repeat the process with remaining dough.
Bake in preheated oven for 9 to 11 minutes. Allow to cool 5 minutes on baking sheet before transferring to a wire rack to cool.
Cool completely then frost.
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.
