Put your softened butter in your mug and put it in your microwave for 10 seconds. It should be melted or partially melted. Mix in the sugars, vanilla extract and egg white gently. Then fold in your flour and drinking chocolate powder, keep folding until all ingredients are well combined. Do this gently so you don't lose air.
Microwave on full power for 2 minutes in a 900 watt convection oven (adjust if your have a different power oven) or until springy to the touch. Insert a knife into the cake and if it comes out clean your cake is done. Decorate with any toppings or sprinkles you want and dig in.