Place butter in the mug and melt in microwave (about 30 seconds).
Add both sugars and stir together.
Add the egg yolks, mixing thoroughly.
Add flour, salt and cocoa powder and stir until combined.
Divide the mix between two mugs.
Cook in the microwave separately for about 45 seconds - 1 minute. (This timing is based on my 1200W microwave so check the wattage of your timing may vary)
Serve with a few marshmallows on top and enjoy! (if you pop them in the microwave for 15 seconds they puff up :)