Mix the oats, flour, baking soda, and salt in a small bowl and set it aside. Add the butter and sugar to another, slightly larger bowl. Cream for 1 minute.

To this add the egg yolk and vanilla. When you mix this in, be sure not to overmix. After the eggs are added, do not mix too much, only to incorporate. Next, add the flour mixture and again combine to incorporate. Last is the chocolate chips. They can easily be mixed in by hand or quickly with the hand mixer.

Scoop the cookies onto your baking sheet and let them chill in the fridge for at least 1 hour up to 2 days. This will give you the chewy cookie texture you are looking for. Preheat the oven to 350 degrees and bake the cookies for 12-14 minutes. The edges should be a very light golden color. If they are baked to long, they will be crispy instead of soft.