Preheat the oven to 350°F(177C°). Grease regular size cupcake pan and set aside.
Mix flour, ground cinnamon, cornstarch and baking soda in a big bowl. Set aside.
Place the butter in a large bowl and beat on medium speed for 1 to 2 minutes until it is soft with a hand-held mixer or with paddle attachment if using a stand-mixer. Add in the brown and white sugar and cream on medium speed until light and fluffy. Add the egg, mix well. Then, add flour mixture and mix until it is totally incorporated. Finally, add in the rolled oats and mix well. During these steps, stop the machine and scrape down the sides and bottom of the bowl with rubber spatula as needed to be sure they are all mixed.
The dough will be very sticky. (If you like you can keep the dough in the fridge for almost 30 minutes but actually, you don’t need to as it is easy to give a cup shape even the dough is sticky) Use a spoon to fill the muffin tin. Divide the dough into equally 10 pieces(a little more than 2 ½ tablespoons-55grams). Press them onto bottom and up sides to give a cup shape with a spoon.
Bake for 14 minutes or until they are golden brown. When the cookie cups are out of the oven, they’ll be puffed. Gently press the center with a small glass to give their cup shape again. Place a parchment paper under the glass to avoid the risk of sticking. Remove the cookie cups from the edges with a toothpick after they cool in the pan for 15 minutes.