Preheat oven to 425 degrees or 400 convection with rack in center of oven.
In the bowl of a stand mixer, or a large mixing bowl, place softened butter, and both sugars. Blend until there are no visible chunks of butter, about 1 minute on low or medium low.
Add egg and mix until smooth on low or medium low.
Stop mixer and add flour, cornstarch, oats, baking soda, salt and cinnamon. Mix on low until just incorporated.
Add the raisins, and mix until just combined.
Measure out 3 oz portions using a kitchen scale, or a little smaller than a baseball. Do not compact the dough or use a cookie scoop and stack the cookie high so it will still be thick after baking.
Place the cookies on a prepared cookie sheet, lined with aluminum foil.
Bake for 5-6 minutes or until the cookie is barely set and golden on top.
Remove from oven and let sit for at least 30-60 minutes to allow the cookie to set up.