Preheat oven to 300 degrees F.
Add butter to a large mixer bowl and beat butter on medium speed until creamy, about 1 minute. Add sugar and continue to mix until combined. Lower mixer to low and gradually add in flour. When flour is just combined raise to medium and mix one minute.
Spread evenly into a 9 x 13 inch baking pan.
Combine 1/3 cup of sugar an cinnamon and sprinkle over shortbread.
Bake for 25-30 minutes.
Allow to cool completely in the pan. Cut into 2 inch squares to serve.