Preheat the oven to 350°.
Cream together the butter, shortening and sugar.
Scrape the sides of the bowl and beat in the egg and vanilla until well combined.
Add in the flour, baking powder and cornstarch.
Scoop the dough into balls and roll in the cinnamon and sugar mixture. (Hint: put the cinnamon and sugar in a baggie and shake a few cookie dough balls at a time! My kids loved helping with this!)
Place the cinnamon and sugar covered cookie dough balls onto a greased cookie sheet.
Bake at 350° for 8 minutes, or until the center of the cookie no longer looks glossy.
The cookies will still be very much in a ball shape when they come out of the oven.
Use the back of a drinking glass to gently flatten the cookies.
Let the cookies cool completely on the pan before moving or serving.
Store in an air tight container.