Prep: Preheat oven to 350 degrees. Line a 9×13 baking pan with parchment (hang some over the sides so it’s easier to remove the cookie bars) – or lightly spray with nonstick cooking spray.
Make the crust: To a large bowl, add melted butter and graham cracker crumbs. Stir to combine.
Transfer graham cracker mixture to the baking dish and press evenly into the bottom to form the crust. Bake 8 minutes. Remove from the oven and cool 10 minutes. (reserve the bowl – you’ll use it in the next step).
Toppings: To the same bowl, blend the can of sweetened condensed milk, salt and vanilla extract together. Set aside.
Layering: Evenly sprinkle all of the chocolate chips over the top of the crust and 1 cup of the marshmallows.
Drizzle reserved sweetened condensed milk blend over the top.
Bake: 12 minutes. Remove from the oven, sprinkle the remaining marshmallows over the top and bake another 10-12 minutes or until marshmallows are golden. Optional: broil 1 minute for darker golden brown color.
Cool: Remove from the oven, cool 15 minutes then gently press mini Hershey bars into the top of the cookie bar. NOTE: if you add the Hersheys before it’s cooled, they’ll melt into the bars.
Slice and eat: Cool completely before slicing (optional refrigerate) then use a sharp knife to slice into squares.