In a mixing bowl, combine graham cracker crumbs, melted unsalted butter, brown sugar, granulated sugar, egg, all-purpose flour, and kosher salt. Mix until a cohesive dough forms.

Line an 8x8-inch baking dish with parchment paper, leaving overhang. Press half of the prepared dough evenly into the bottom.

Evenly distribute semi-sweet chocolate bars over the pressed dough layer, then spread peanut butter, followed by marshmallow cream on top.

Crumble the remaining dough over the marshmallow layer, sealing in the fillings.

Preheat oven to 350°F (175°C). Bake for about 30 minutes, until golden brown on top.

Let cool at room temperature for 4-6 hours, then slice into squares.