Combine butter and chocolate chips in a large mug (**OR divide all ingredients evenly between 2 regular sized mugs.)
Microwave on high power for 30 seconds; remove from microwave and stir until completely melted.
Add eggs, sugar, cocoa powder, and vanilla. Stir well.
Add flour and baking powder and stir until completely incorporated.
Microwave on high power for 1 minute. Stop microwave and stir, reaching spoon all the way to the bottom of the mug.
Microwave 30 more seconds, then stop microwave and microwave 15-30 seconds more.
Top with a handful of chocolate chips and whipped cream as desired.
Notes
I made this in an extra large mug, but you will likely choose to divide ingredients evenly between two regular sized mugs. IF you choose to combine them all in one mug, I would actually recommend sharing!