In a large microwave safe mug, mix together the flour, cocoa powder, sugar and baking soda until combined. Add in the milk, oil and vanilla extract, and mix until smooth.
Sprinkle the top with chocolate chips and microwave for 1 minute, or until the center of the cake is set.
Place the marshmallows on a baking sheet and broil for around 20 seconds.
Place the marshmallows and crushed graham crackers on top of the mug cake. Allow to cool slightly before eating.