Preheat oven to 350°F. Line a 9x13 inch baking pan with foil or parchment paper and spray with cooking spray.
In a medium bowl, combine graham cracker crumbs and melted butter. Stir until all crumbs are moistened and mixture holds together when squeezed.
Press the crumb mixture firmly into the bottom of the prepared pan, going up the sides about 1 inch if desired.
Bake the crust for 8 minutes until it just starts to firm up. Remove from oven.
Immediately sprinkle chocolate chips evenly over the hot crust, then top with an even layer of mini marshmallows.
Return to oven and bake for 18-20 minutes until marshmallows are puffed and golden brown on top.
Cool completely on a wire rack, then refrigerate for at least 1 hour before cutting into squares.