In a large bowl, cream together butter and brown sugar until smooth. Beat in the egg and vanilla until well blended. Combine the flour, cornstarch, salt, and cinnamon; stir into the creamed mixture.

Stir in the white chocolate and oats. Refrigerate for at least two hours or up to five days in a bowl covered in plastic wrap.

Preheat oven to 350 degrees F (175 degrees C).

Scoop into tablespoon-sized balls and place them onto a lined baking sheet.

Bake for 10-12 minutes in the preheated oven, or until the outsides are set and golden brown and the middle is just beginning to set. Don't over-bake.

Cool on the baking sheet for 10 minutes, and then transfer to a wire rack to finish cooling. They should be soft. Cool completely and store in an airtight container for up to 5 days.