Design Eat Repeat

0 Preheat oven to 350ºF. While preheating, be sure to read tips & tricks in post for recipe success.
21 In a medium-sized mixing bowl, beat the softened unsalted butter ([adjustable]½ cup, i.e. 1 stick) and peanut butter (¾ cup[/adjustable]) on medium speed for 20-30 seconds until combined.
26 Add in the light brown sugar ([adjustable]¾ cup) and granulated sugar (¼ cup[/adjustable]) and continue beating on medium for another 20-30 seconds until creamy. Do not over mix.
22 While the mixer is on low, add in the egg ([adjustable]1 large), extra egg yolk (1 large), vanilla extract (1 teaspoon), baking soda (½ teaspoon), salt (¼ teaspoon), and cornstarch (1 tablespoon[/adjustable]). Stir until combined. Slowly add in all-purpose flour ([adjustable]1 ½ cups[/adjustable]) and mix until just incorporated.
28 Mix in chocolate chips ([adjustable]1 cup) and marshmallows (1 cup[/adjustable]).
24 Grease and line a 9×13 light-colored metal pan with parchment paper (I like to create a parchment sling so I can lift the bars out of the pan to cut). Spread dough into an even layer in pan. 
25 If desired, press on extra chocolate chips or peanut butter cups into the top.
27 Bake for 16-18 minutes or until edges are lightly brown, marshmallows are lightly toasted, and the center no longer looks wet (some ovens may need more or less time). Allow the bars to cool for at least 30 minutes before cutting, as they will fall apart when warm. For the best slice, I recommend refrigerating the bars for about 1 hour before slicing.