01 -
Stir the graham cracker crumbs with melted butter, sugar, and a pinch of salt until the texture resembles wet sand. Press the mixture firmly into the bottom of a lined 9-by-13-inch pan and bake at 350°F (175°C) for 8 minutes, just until it smells toasty.
02 -
Combine the chopped chocolate and butter in a heatproof bowl set over simmering water. Stir until smooth, then whisk in the sweetened condensed milk, hot cocoa powder, vanilla, and salt until the mixture is glossy and thick.
03 -
Pour the chocolate filling over the warm crust and spread it into an even layer. Bake for 18 to 20 minutes until the center is set but still slightly soft. It will firm up as it cools.
04 -
Scatter the mini marshmallows evenly across the chocolate layer. Switch the oven to broil and return the pan for 1 to 2 minutes, watching constantly, until the tops are deeply golden with a few charred spots.
05 -
Let the bars rest at room temperature for 30 minutes, then refrigerate for at least 1 hour before lifting them from the pan and slicing with a hot knife.