In a small bowl mix together the flour, cocoa powder, baking powder, sugar, and pinch of salt.
Add in the melted butter or oil and milk, and stir until mixed.
Batter will be thick, don't be alarmed. :)
Spray your mug with cooking spray and spoon in half the batter.
Spread out the batter with the back of your spoon or a knife.
Make a little indent in the middle of the batter, and fill it with the chocolate chips and marshmallows.
Spoon the rest of the chocolate batter over the fillings, and gently spread it out to cover the marshmallows and chocolate chips.
Microwave for 1 minute.
My mug cake was perfectly done at 1 minute; if yours needs a bit more time, add 15 seconds at a time and watch it.
Top with whipped cream, coconut whipped cream, or even extra marshmallows!