Preheat oven to 350 degrees F. Lightly grease a 9X13-inch baking pan with cooking spray or line with parchment and lightly grease parchment and set aside.
In a large bowl, add the butter, granulated sugar, brown sugar, egg, egg yolk, vanilla, baking soda and salt, and mix by hand or with an electric mixer until well-combined.
Add the flour, oats and chocolate chips and mix until evenly combined.
Press the batter evenly in the prepared pan and bake for 18-20 minutes. Don't over bake! These bars are best under baked just a tiny bit.
Notes
Butter: to take it up a notch, try browning the butter first. Amazing! Also, I always use salted butter.Doubling: this recipe doubles great to fit in a half sheet pan (11X17-inch rimmed baking sheet).Update: the original bar recipe posted in 2008 called for 1 3/4 cups flour, but I've long since cut the flour amount back to 1 1/2 cups for softer, chewier bars.