In a medium bowl whisk together the all-purpose flour, baking soda and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat together the butter, brown sugar and granulated sugar until creamy. Add the egg, egg yolk and vanilla and mix until well combined, scraping down the sides of the bowl as needed.
With the mixer on low speed, mix in the flour mixture until just combined. Then mix in the oats on low speed, scraping down the sides of the bowl as needed. Finally, mix in the chocolate chips on low speed.
Cookies may be baked right away, but for best results cover and refrigerate the dough for 1 hour (and up to 24 hours) before baking. (If chilling the dough for 2+ hours, you may need to let the cookie dough sit at room temperature for about 10 minutes to soften up a little before rolling the cookie dough balls.)
When you are ready to bake the cookies, preheat oven to 350° F and line your baking sheet(s) with parchment paper.
Scoop rounded tablespoon-sized balls of dough, roll into balls, and place on the parchment-lined baking sheets, 3 inches apart.
Bake for 9-11 minutes, until cookies are almost set in the center. Be careful not to over-bake. For the softest cookies you want to slightly under-bake. If baking two pans of cookies at once, rotate the pans halfway through the baking time.
Let cookies cool on baking sheet for 5-10 minutes, then remove to a wire rack to cool completely.