Crush cookies and add 1 tablespoon butter. Microwave for 10 seconds to melt, stir and press into the bottom of your mug.
In a separate bowl, microwave 3 tablespoons of butter for 10 seconds to melt.
Add in egg and milk and lightly beat with a fork until combined.
Add dry ingredients and stir with a fork until mixed through.
Pour chocolate batter into mug on top of the cookie base.
Top with marshmallow fluff or marshmallows.
Place mug in the microwave and cook for 1 minute on medium heat.
Finish off the mug cake by toasting the marshmallow topping under the grill or use a blow torch.