Preheat the oven to 350° F.
Prepare a 9x13-inch baking dish by lining the dish with parchment paper (or foil). Spray the parchment paper with cooking spray and set aside.
Crush the graham crackers with a rolling pin in a ziplock bag (You could also use a food processor).
In a small bowl melt the butter in the microwave for 30 seconds. Then stir, and microwave another 30 seconds, or until butter is all melted.
Add the crushed graham crackers and sugar to a large bowl. Stir in the melted butter. Mix until fully combined and all of the crumbs are coated in the butter. Pour into the prepared baking dish and press firmly with the bottom of a measuring cup to create a smooth layer (you can also use clean fingers to help press the crust into the pan).
Bake for 8 minutes.
Remove from the oven and sprinkle the chocolate chips evenly over the hot crust.
Add the mini marshmallows in a layer over the top of the chocolate chips.
Put back in the oven and bake for 12-17 more minutes, or until the marshmallows are puffed and light golden brown.
Remove from the oven and place on a cooling rack to cool down.
When cooled completely, or just a little warm, slice into 15 bars (or whatever size and amount you need).