Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper and position the oven rack in the center.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon. Set aside.
In a large mixing bowl, beat the butter, brown sugar, and granulated sugar for 2 to 3 minutes, until lighter and fluffy.
Add the eggs, egg yolk, and vanilla extract. Beat until smooth, scraping down the sides of the bowl as needed.
Add the dry ingredients and mix on low speed just until no flour streaks remain. Do not overmix.
Fold in the rolled oats and chocolate chips with a spatula until evenly combined.
Scoop about 2 tablespoons of dough per cookie onto the prepared baking sheets, leaving around 2 inches between each cookie.
Bake for 13 to 15 minutes, until the edges are lightly golden and set but the centers still look soft.
Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.