Cream together the butter, brown sugar, and granulated sugar over medium speed until light and fluffy, about 5 minutes.
While the mixture is creaming, whisk together the flour, baking soda, baking powder, salt, cinnamon, and nutmeg in a separate bowl.
Once the butter and sugars are creamed, add the eggs and vanilla and beat to combine, then scrape down the sides. With the mixer on low speed, spoon in the dry ingredients until combined, then add in the oats last and give one more mix.
Place the bowl and dough in the fridge to rest for at least two hours. Do not skip this step*.
Preheat the oven to 350℉/177℃ and line 2 baking pans with parchment paper.
Using a 1.5 inch cookie scoop, press the dough into the scoop with the palm of your hand, add the cookies to the baking pans, leaving as much space in between as possible.
Bake for 8 minutes. At the 8 minute mark, “scoot” your cookies to make them perfectly round. You can do this with a cookie cutter (or anything with a round opening - e.g. sour cream container) larger than your baked cookie or with a fork. Simply surround your cookie with the cutter and move it around so the sides round back up. With the fork, place the fork on its side and use the tines to round out any uneven edges. Because we’re making sandwiches, we want these to be as evenly round as possible.
Once shaped, return to the oven for about 2-4 minutes. The middles will still seem very underdone - this is what we want. Remove the pan from the oven, scoot any deformed edges again, and place the pan on a wire rack to cool. Allow the cookies to cool completely on the pan (this will finish setting the center).