Preheat your oven to 350°F. Line a large baking sheet with parchment paper, and set aside.
In a medium bowl, combine flour, baking soda, baking powder, cornstarch, salt, and cinnamon. Whisk to combine, and set aside.
In a large bowl, cream together butter, brown sugar, and granulated sugar until you no longer see any dry sugar crystals (about 2 minutes with a stand mixer on medium speed). Add your egg and vanilla, and whisk to combine.
Add the flour mixture to the butter mix, and stir together until just combined. Fold in your oats as well as any add-ins you'd like (see post above for ideas!) Take care not to over mix!
Portion and roll your cookie dough into 12 balls (about 2 tablespoons each). Arrange on your parchment lined baking sheet, leaving about 1 inch of space between each ball to give them room to expand. * If you're having trouble rolling the dough, pop it in the fridge for 10 to 20 minutes, or until it's firm enough to work with!
Bake in the center of your preheated 350°F oven for 10 to 11 minutes for soft cookies or 14 to 15 minutes for crispy cookies. Soft cookies should just be golden brown on the edges, while the center should appear loose and raw (don't worry, it sets up as it cools!) Crispy cookies should be matte on top and golden brown throughout. Once baked, cool the cookies for 5 minutes on the baking sheet, then transfer to a wire rack to cool completely!