Preheat your oven to 350°F (175°C) and prepare a 9x13-inch baking pan with cooking spray.

In a mixing bowl, combine crushed graham cracker sheets, melted unsalted butter, and granulated sugar until it resembles wet sand. Press this mixture firmly into the bottom of the prepared pan.

In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. In another bowl, cream together unsalted butter and granulated sugar until fluffy. Add a large egg and vanilla extract, mixing until well incorporated. Gradually blend in the flour mixture until just combined.

Spread the prepared cookie layer evenly over the graham cracker crust.

Sprinkle milk chocolate chips and mini marshmallows over the cookie layer, and drizzle sweetened condensed milk on top.

Bake for 22-27 minutes, until edges turn golden brown and center remains slightly soft.

Let the pan cool for at least 30 minutes before slicing into squares.