Place chocolate and butter in a small heatproof bowl, then set in the microwave and heat for 30 seconds. Take it out and stir, then continue to microwave in 20-second intervals, stirring between each interval until melted and smooth.
Add sugar to the bowl and mix until combined. Add egg and whisk vigorously until completely smooth. Lastly, add flour and mix just until combined and lump-free, but don’t over mix.
Scrape batter into your favorite mug (the mug should be bigger in capacity than the batter to avoid overflowing). Microwave for 1 minute at 700-750 W. If it’s undercooked, heat for a few seconds longer. I like it when it’s not fully cooked and there are still molten parts. Let stand for 2 minutes, then dig in.