Line an 8" square baking pan with foil, leaving an overhang. Spray lightly with non-stick spray. Set aside.
In a medium sized saucepan1, combine the chocolate chips and sweetened condensed milk. Melt over medium heat, stirring constantly until completely melted, then remove from heat. Mixture will be very thick.
Add the crushed graham crackers, reserving a small amount (about 2 to 3 Tablespoons) for topping, and the marshmallows. Fold gently with a large spatula to combine.
Pour the fudge mixture into the prepared pan and spread evenly with a spatula or the back of a spoon. Sprinkle reserved crushed graham crackers over the top of the fudge, pressing down lightly with your hands to insure the graham cracker bits stick. Top with a few more mini marshmallows, pushing each one into the top of the fudge. Chill in the refrigerator for at least 2 hours.
When fudge is completely cool, lift foil out of the pan and, using a very sharp knife, cut the fudge into desired size pieces. I prefer 36 small bites. Store fudge in an airtight container2 in the refrigerator up to 2 weeks.
Using a saucepan: alternatively, you can heat the chips and milk in the microwave. Heat on HIGH in 20-second increments, stirring after each, until completely melted.
Leftovers: graham cracker pieces will lose their crunch over time, especially in the refrigerator.