In a standard size coffee mug mix together the flour, cocoa powder, sugar, baking powder, baking soda, espresso powder and salt.
Add the milk, oil and vanille extract and use a fork or a small whisk to completely combine. If adding in chocolate chips, peanut butter, etc. add in now!
Place the mug in your microwave for 60 to 90 seconds or until the cake has puffed up and is mostly dry to the touch. Keep an eye on the cake as it cooks as microwaves can vary a lot in power!
Top with whipped cream or ice cream or eat as is! Enjoy!