Line an 8" x 8" baking pan with foil, leaving an overhang. Spray with non-stick spray. Set aside.
In a medium sized saucepan, combine the chocolate chips and sweetened condensed milk. Heat over medium heat, stirring constantly until completely melted. Mixture will be very thick. Remove from heat.*
Add the crushed graham crackers, reserving a small amount (about 3 Tablespoons) for topping, and the marshmallows. Stir to combine.
Pour fudge mixture into 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 again with the spatula to ensure 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 64 tiny bites. Store fudge in an airtight container in the fridge up to 2 weeks.
*Alternatively, you can heat the chips and milk in the microwave. Heat on HIGH in 20-second increments, stirring after each, until completely melted.