Preheat the oven to 350°F. Lightly grease an 8- or 9-inch round cake pan. For easier cleanup, you can line the pan with foil first and grease the foil. You can also use a lined quarter sheet pan for a thinner, more classic dessert nachos style.
Break the graham crackers into squares or rough pieces. Arrange them in the pan in a mostly even layer, overlapping slightly and filling in gaps with smaller pieces. You want the base to look layered and a little rustic, like nachos, rather than perfectly flat.
Add the milk chocolate chips and butter to a microwave-safe bowl. Microwave for 20 seconds, then stir. Continue microwaving in 15- to 20-second bursts, stirring well between each one, until the chocolate is mostly melted and smooth. If a few small pieces remain, keep stirring instead of microwaving longer so the chocolate does not overheat.
Drizzle the warm chocolate sauce over the graham crackers.
Sprinkle the optional extra chocolate chips over the top, if using. Add the mini marshmallows evenly over the graham crackers and chocolate.
Bake for 5 to 7 minutes, or until the marshmallows are puffed and the chocolate is melty.
Turn the oven to broil and broil for 30 to 90 seconds, watching constantly, until the marshmallows are golden brown.
Remove from the oven. Add extra graham cracker pieces or crumbs on top if desired. Serve warm.