Preheat the oven to 350 degrees F (177 C). Line a baking tray(s) with parchment paper.
In a large bowl (or the bowl of a stand mixer), cream the butter and brown sugar together until smooth and creamy using a hand/stand mixer. Add the egg and mix until light and fluffy.
Add the starch, salt, vanilla, cinnamon, and baking soda and mix until combined. Add the rolled oats and oat flour and beat until everything is well mixed.
Scape the sides of the bowl with a spatula. Add whatever mix-in(s) of choice and mix until well combined (I split my dough into three equal parts and added ⅓ cup each of raisins, butterscotch chips, and chopped caramels separately into the the three dough portions). The dough should be kind of sticky.
Using a cookie scoop, scoop dough balls onto the lined baking sheet(s). If you want to roll the dough into perfect balls with your hands, (which I did not do and is totally optional) chill the balls in the freezer for 15 minutes before rolling, so that your hands won't get messy. Bake for 8-10 minutes or until the edges are just slightly golden brown. Sometimes they don't even look like they're done baking, but I don't recommend baking any longer than 10 minutes (unless you are making big cookies). Let the baked cookies cool for at least 5 minutes before eating. Enjoy!
