Heat oven to 325 degrees. Prepare two cookie sheets by lining with parchment paper, baking mat, or spray with cooking spray. Set aside.
In the bowl of a stand mixer, or in a bowl with a handheld blender, beat together the butter, brown sugar, and sugar. Beat for 1-2 minutes until it's light in color and looks fluffy.
Add in the egg, egg yolk, and vanilla extract. Beat together until combined. Scraping sides of bowl as necessary.
In a separate smaller bowl, add the flour, oats, baking soda, baking powder, and salt. Stir with a whisk. Add half to the wet dough and blend together. Add the other half and blend until just combined. Stir in the desired amount of chocolate chips with a wooden spoon.
Scoop out 1/4 cup of cookie dough (use a large ice cream scoop or 1/4 cup measurer) and roll into a ball. Place 5 cookie dough balls per cookie sheet.
Bake for 14-16 minutes. Edges should be very pale golden brown while the center should still look somewhat underdone. After baking, let cool on the cookie sheet for about 20 minutes. The cookie will come together and finish baking.
Store leftovers in a covered container on the counter top. These reheat really well in the microwave if you have leftovers.
I always use light brown sugar but you can use dark brown sugar as well. Whichever you prefer. Dark brown sugar has a stronger flavor due to the higher molasses in it.
I always use salted butter when baking plus the additional salt called for in recipe. The cookies are sweet so I want that flavor that the salt brings.
I add about 1 1/2 cups chocolate chips because I don't love a ton of chocolate in my cookies. But if you do, then add up to 3 cups of chocolate chips. You can use milk chocolate, semi-sweet chocolate, or even dark chocolate.
All ovens are so different so set the cook time for 13 minutes and then go from there. Mine were done right around 14 1/2 minutes about.