Heat the oven to 350°f.
Combine the butter and sugars in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 2 to 3 minutes, until fluffy and pale yellow in color. Scrape down the sides of the bowl with a spatula. On low speed, add the egg yolk and increase the speed to medium-high and beat for 1 to 2 minutes, until the sugar granules fully dissolve and the mixture is a pale white.
On low speed, add the flour, oats, baking powder, baking soda, and salt. Mix for a minute, until your dough comes together and any remnants of dry ingredients have been incorporated. The dough will be a slightly fluffy, fatty mixture in comparison to your average cookie dough. Scrape down the sides of the bowl.
Pam-spray a quarter sheet pan and line with parchment, or just line the pan with a silpat. Plop the cookie dough in the center of the pan and, with a spatula, spread it out until it is 1/4 inch thick. The dough won’t end up covering the entire pan; this is ok.
Bake for 15 minutes, or until it resembles an oatmeal cookie-caramelized on top and puffed slightly but set firmly. Cool completely before using. Wrapped well in plastic, the oat cookie will keep fresh in the fridge for up to 1 week.