Step 1

Preheat oven to 350°F.  In the bowl of a stand mixer, use the paddle attachment to cream together the softened butter and sugar on medium-high speed until light and fluffy, about 3 minutes.  Add egg and vanilla, and mix until combined, about 1 minute.

Step 2

In a separate bowl, whisk together the flour, cinnamon, baking soda, cornstarch and salt until combined.  Add in this flour mixture gradually to the butter mixture, and mix on medium-low speed until combined.  Add the oats, and continue mixing on medium-low speed until combined.  If you are using any add-ins (up to 2 cups), fold them in on low-speed until just combined.  Do not overmix.

Step 3

Use a heaping small cookie scoop (a little over 1 Tablespoon) or a spoon to shape the dough into balls, and place them on a parchment-covered baking sheet.  Chill in the refrigerator for at least 10-20 minutes.

Step 4

Bake for 10-12 minutes until the edges are just set and are lightly golden. Cool cookies on the baking sheet for about 5 minutes, then transfer to a wire rack to finish cooling.

Step 5

Serve immediately or store in a sealed container for up to 1 week. You can also freeze the cookies for up to 3-4 months.