Place the rack in the middle of the oven and preheat to 325° F/165° C.

In the bowl of the stand mixer, beat the butter, brown sugar, and granulated sugar until smooth and creamy. Add the egg, egg yolk, and vanilla, one at a time, beating well between.

In the medium mixing bowl, whisk together the flour, oats, baking powder, baking soda, and salt. Add to the butter mixture and beat until everything is incorporated. Scrape down the sides of the bowl and mix in the chocolate chips.

Shape the dough into rounded teaspoon-size balls and place them about 2 inches apart on the prepared baking sheet, and gently press each down. Bake in the preheated oven until the cookies start to brown at the edges, about 12 to 15 minutes. Cool on the baking sheet about 5 minutes, then transfer to a wire rack to cool completely. Repeat with the remaining dough. Store in an airtight container.