Whisk together the cake flour, corn starch, baking powder, and salt, and set aside. In a large bowl or the bowl of a stand mixer, beat the butter, shortening, and sugar until light and fluffy. Add the egg, egg white, vanilla extract, lemon extract, and lemon zest, and beat until smooth. Add the flour mixture and mix until just combined. Wrap in plastic wrap and refrigerate one hour (or freeze 20 minutes if you're impatient like me).