Design Eat Repeat

0 Preheat oven to 350ºF. While preheating, be sure to read tips & tricks in post above for recipe sucess.
26 In a large mixing bowl, beat together unsalted butter [adjustable](½ cup, i.e. 1 stick), light brown sugar (¾ cup), and granulated sugar (¼ cup)[/adjustable] on medium speed for 30-60 seconds, or until combined. Mix in creamy peanut butter (¾ cup).
22 Stir in the [adjustable]1 whole egg, 1 extra egg yolk, and vanilla extract (1 teaspoon), followed by the baking soda (½ teaspoon) and salt (¼ teaspoon)[/adjustable].
23 Add all-purpose flour [adjustable](1½ cups) and cornstarch (1 tablespoon) [/adjustable]and mix until combined.
28 Add semi-sweet chocolate chips [adjustable](½ cup) and peanut butter chips (½ cup)[/adjustable] . Mix the dough until well-combined.
24 Generously grease and line a light-colored metal 9x13 pan with parchment paper. Spread dough into an even layer in pan. If desired, press a few extra chocolate chips on the top for added presentation.
27 Bake for 14-16 minutes, or until edges are slightly golden brown. Do not overbake; it might look a little under-baked, but the residual heat from the pan will continue to bake the cookie even after you take it out of the oven. 
29 Cool completely before cutting - this is important! If you cut while they are warm, they will crumble.
30 Store in an airtight container for 3-4 days or freeze up to 1 month.