Combine 3 tablespoons butter and 2 tablespoons chocolate chips in a small bowl, then melt in the microwave for 20-30 seconds. Set aside.
In another bowl, combine remaining 1/2 tablespoon melted butter with 2 tablespoons of graham cracker crumbs and stir until moistened.
Press graham crumbs into the bottom of your mug.
In a bowl. whisk egg, sugar and vanilla until smooth.
Add in flour, baking powder, salt and cocoa, stir until a thick batter forms.
Stream in melted butter and chocolate, mixing to combine.
Fold in remaining 1 tablespoon chocolate chips.
Add half of the mixture on top of the graham crust in the mug.
Then add a few mini marshmallows (I did about 10) or you can add a scoop of marshmallow fluff instead.
Add remaining batter on top, then pop in the microwave for 1 minute and 20 seconds (I have a 1000 watt microwave).
Remove and top with a hearty scoop of marshmallow fluff.
Put back in the microwave for 5 seconds to melt it and make it ooey gooey.
Carefully remove from microwave (I set mine on a plate since the marshmallows can be messy) and sprinkle with graham cracker crumbs!
Serve warm with a spoon and dig in!