Pour 1 cup Lucky Charms cereal and milk in a bowl and place in fridge for an hour. After chilling, pour milk through a strainer into a saucepan, leaving cereal behind.
Set saucepan over medium heat and once milk is hot (but not boiling), add chopped chocolate and whisk to combine. Continue whisking until chocolate has completely melted into milk, making sure you don't let milk boil.
Remove from heat and stir in marshmallow fluff.
Pour hot chocolate into mugs and top each mug with ¼ cup marshmallows.