In a microwave-safe mug, combine the all-purpose flour, granulated sugar, cocoa powder, baking powder, and salt. Mix thoroughly until there are no lumps.
Add the milk, peanut butter, vegetable oil, and vanilla extract to the dry ingredients in the mug. Use a fork to whisk everything together until you have a smooth batter.
If desired, stir in some chocolate chips for an extra chocolatey treat.
Microwave the mug cake on high for 1 minute and 20 seconds. (Microwave times may vary, so keep an eye on it and adjust accordingly.)
Once it's done, let the mug cake cool for a minute before digging in; it will be very hot!
Enjoy it straight from the mug or transfer it to a plate to show off the fluffy texture!
Notes
Microwave times may vary, so keep an eye on it and adjust accordingly.