Preheat oven to 350 degrees F.Line a large rimmed baking sheet with parchment paper.Using an electric mixer fitted with the paddle attachment, cream together the butter and sugar until light and fluffy.In another bowl, combine the flour, 1 cup of oats, salt and cinnamon.Add to the butter mixture and stir until combined.
Finely grind the remaining 2/3 cup of oats in a food processor.Dust your rolling surface with some of the ground oats.Divide the shortbread dough in half and roll out each into a circle about 8 inches in diameter and 3/8 inch thick.Sprinkle the remaining oatmeal over the top of the dough and press it into the surface with the rolling pin.Using a sharp knife, cut each circle into 8 wedges.Prick each wedge all over with the tines of a fork.Transfer to the prepared baking sheet and bake until golden brown, approximately 18-20 minutes.Cool on a rack and then store in an airtight container.
Source: Sarah Leah Chase - Cold Weather Cooking - 1990