Crust:

2 cups graham cracker crumbs (from about 16 crackers)
1/2 cup unsalted butter, melted, plus more for pan
Fudge:
3 cups semi-sweet chocolate chips
1 (14-ounce) can sweetened condensed milk
1/4 cup unsalted butter
1 (10-ounce) bag mini marshmallows
Heat oven to 350° F. Line a 9×13-inch baking pan with parchment paper and butter lightly.
In a mixing bowl, stir together graham cracker crumbs and melted butter. Press evenly into the prepared pan. Bake until set, about 8 minutes. Heat broiler.
In a medium saucepan, combine butter, sweetened condensed milk and chocolate chips. Heat over medium-low, stirring, until the mixture is completely melted and smooth, 6 to 8 minutes. (The mixture will be very thick.)
Spoon mixture over crust and use a spatula to spread into an even layer. Sprinkle marshmallows over, pressing lightly to make them stick to the fudge layer. Broil until dark golden brown, about 2 minutes. (Keep an eye on it, it can burn very quickly.) Let stand at room temperature until set, about 2 hours, before cutting into pieces.