Preheat the oven to 350 F and place a rack in the center of the oven. Line a half size (13" x 18") baking sheet with a silicone baking mat or a sheet of parchment paper.
Brown the butter by adding the butter into a medium sauce pan. Place over medium heat and stir with a rubber spatula. Continue cooking until brown bits start to appear. We want all of those brown bits but we don’t want it to burn so turn down the heat if necessary. After it gets quiet and foamy continue to cook until brown bits start to foam up.
Once browned, remove from the heat and pour it into a heat safe mixing bowl. Add in the cinnamon and stir to combine. Let cool for five minutes.
Whisk in the brown sugar, white sugar, salt, and vanilla. Add in the egg and egg yolk and whisk again.
Pour in the flour, starch, baking soda, baking powder, and oats. Fold to combine.
Dollop the cookie dough over the lined baking sheet.
Use an offset spatula or a silicone spatula to carefully press the cookie dough over the lined baking sheet. This takes a bit of patience but I promise it'll work.
Bake the cookie bar for 6 minutes, then rotate and bake for another 4 minutes. Don't over bake this! It'll continue to bake as it cools on the baking sheet.
Once baked, take the pan out of the oven, and leave it to cool completely before trying to move it.