Heat the oven to 350 degrees and line a mini muffin tin with paper liners.
Melt the butter in a medium saucepan over low heat. Remove from heat and stir in the sugar until combined.
Add the eggs one at a time, mixing well after each, then stir in the vanilla extract.
Whisk in the cocoa powder, flour, and salt until the batter is smooth and no dry streaks remain.
Fold in the graham cracker crumbs, mini marshmallows, and chocolate chips until evenly distributed.
Spoon the batter into the prepared muffin cups, filling each about three-quarters full.
Bake for 12 to 14 minutes until the tops look set but the centers still feel slightly soft.
Cool the bites in the tin for 10 minutes before moving them to a rack.