Heat oven to 350°F (175°C). Grease a 9x13-inch metal baking pan and line with a parchment sling.
Stir graham crumbs, melted butter, brown sugar, and cinnamon until sandy.
Press a thin, even layer into the bottom of the pan. Don’t pack it rock-hard—just gently compact.
In a large bowl, whisk flour, sugar, cocoa, baking powder, baking soda, and salt.
In another bowl, whisk milk, oil, eggs, and vanilla until smooth.
Pour wet into dry. Whisk just until combined.
Stream in hot coffee and whisk until silky and pourable. The batter will look thin—that’s right.
Pour batter over the graham layer. Tap the pan to release bubbles.
Bake 22–26 minutes until the center springs back and a toothpick shows a few moist crumbs.
Let it cool 10 minutes. Leave oven on.
Scatter chocolate chips evenly over the warm cake. They’ll start to melt.
Top with a thick blanket of mini marshmallows. Cover edge to edge.
Switch oven to broil. Move the rack to the upper third.
Broil the cake 30–90 seconds until the marshmallows turn golden with a few charred spots. Watch like a hawk.
Remove and sprinkle a little flaky salt for contrast.
Let it sit 8–10 minutes so the chocolate sets slightly but stays gooey.
Warm a knife under hot water and wipe between cuts for cleaner slices.
Serve as-is or with vanilla ice cream for a showstopper dessert.
