Preheat the oven to 350°F (175°C) and coat 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 into the bottom of your prepared pan.

In another bowl, whisk together all-purpose flour, baking powder, and salt. Cream together softened butter and granulated sugar until light and fluffy, then add in the egg and vanilla extract.

Gently fold the dry flour mixture into the creamed butter mixture and spread it evenly across the crust.

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

Bake for 22-27 minutes until the edges are golden brown and the center is slightly soft.

Let cool in the pan for 20-30 minutes before slicing into squares or rectangles.