Preheat oven to 350 degrees.
In a stand mixer (or you can use a hand mixer), add the brown sugar, granulated sugar, butter, and vanilla extract. Mix on medium speed for 1 to 2 minutes.
Turn mixer on low, slowly add in each egg. Turn mixer back on medium for 1 minute, stopping and scraping down sides as needed. Then turn mixer off.
In a separate bowl, whisk together the oats, flour, baking powder, baking soda, cinnamon and salt.
Turn the mixer on low and slowly add in the flour until it's well incorporated. Don't mix on high or mix for too long, it will make your cookies more tough or more crispy. (If using a hand mixer, you can just manually stir in the flour.)
Place medium sized cookie scoops onto a baking sheet lined with parchment paper.
Bake at 350 degrees for 8-10 minutes.
While they bake, make the icing. In a small bowl whisk together the powdered sugar, vanilla extract, and milk.
Place a sandwich baggy inside a tall glass and wrap the opening of the baggy around the mouth of glass. Scoop the icing into the baggy. Zip it up and cut a tiny tip off the corner for the icing to squeeze out.
Let the cookies cool before icing them. Enjoy!