Preheat oven to 350 F. Grease a 9x13 baking dish with nonstick cooking spray
In a bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
Place the butter and sugar in a stand mixer fitted with a paddle attachment. or in a large mixing bowl using a hand mixer. Beat on medium-high speed until light and fluffy, about 3 minutes.
Add egg + extra yolk, one at a time and mix on low until smooth, about 30 seconds. Beat in vanilla.
Slowly mix in dry ingredients, being careful not to over mix. I usually stop when I still have some flour remnants around the bowl.
Evenly spread the cookie dough to the bottom of the prepared baking dish. I used gloved hands as it takes a lot of handling to smooth out evenly.
Bake for 20-25 minutes, turning the dish halfway through. Cookie base is done when the edges are golden brown and a toothpick inserted in the center comes out clean. Be careful not to over bake. Allow to cool in the baking dish until fully cool (about an hour).