Preheat oven to 350 degrees.
In a large mixing bowl combine butter, shortening and sugar. Beat until fluffy.
Turn mixer down and add egg, vanilla and lemon zest. Add baking powder. Sift in and flour and cornstarch until mixed thoroughly (1-2 minutes).
Scoop dough, using a medium cookie scoop, onto ungreased baking sheets. Roll each scoop of dough into a ball.
Press a glass into each cookie dough ball to flatten. Dip glass in sugar to keep the glass from sticking to the cookie dough.
Bake cookies for 10 minutes. Allow to cool on pan 3-5 minutes before transferring to cooling rack.
Allow cookies to cool completely before frosting.