41.55 ozmilk chocolate Hershey barschopped into pieces*
1 ½cupsmini marshmallows72g
Preheat oven to 350F (175C) and line a 13x9 metal pan with parchment paper or grease and flour the sides and bottom of the pan.
Combine butter, peanut butter, and sugar in a large bowl and use an electric mixer to beat until light, creamy and well-combined.
Add egg/egg yolk and vanilla extract, and stir well.
In a separate bowl, whisk together flour, graham cracker crumbs, baking powder, cornstarch, and salt.
Gradually, on low speed, add flour mixture to peanut butter mixture, pausing occasionally to scrape down sides of bowl.
Add milk and stir until combined.
Fold in chopped Hershey bars and mini marshmallows -- the batter will be thick (I broke a spatula stirring it so proceed with caution!).
Spread batter into prepared ban and bake on 350F (175C) for 25-28 minutes (toothpick inserted in center should come back mostly clean with only a few crumbs)
Allow to cool completely before cutting and serving.
*I usually stir 3 of the hershey bars into the batter and reserve one to press on top of the cookie dough once I've spread it into the pan -- I think it makes the bars look prettier but you can just throw them in the batter if you prefer!