Preheat oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal. Spray lightly with cooking spray.

In a medium bowl, combine graham cracker crumbs, melted butter, and sugar. Press the mixture evenly into the bottom of the prepared baking pan.

Bake the crust for 8-10 minutes, or until lightly golden. Remove from oven.

Sprinkle milk chocolate chips evenly over the warm graham cracker crust.

Return the pan to the oven for 3-5 minutes, or until chocolate is slightly melted. Remove from oven.

Evenly spread 2 cups of mini marshmallows over the melted chocolate. Return to the oven for another 2-3 minutes, or until marshmallows are lightly golden brown and puffed. Watch carefully to prevent burning.

Remove from oven and let cool completely on a wire rack before cutting into squares. This helps the bars set properly.