In a microwave-safe mug (preferably 12oz), combine all the dry ingredients: flour, sugar, cocoa powder, baking powder, and salt. Stir well to ensure everything is evenly mixed.
Add the milk, peanut butter, vegetable oil, and vanilla extract to the dry mixture. Use a fork or small whisk to blend the ingredients until smooth and no lumps remain. If desired, fold in some chocolate chips for added richness.
Microwave the mug on high for about 1 minute and 30 seconds, or until the cake has risen and appears set. Check for doneness; if it's still gooey in the center, microwave in 15-second increments until done.
Let the mug cake cool for a minute before indulging. For an extra treat, top with a dollop of whipped cream, additional peanut butter, or a sprinkle of chopped nuts.
Notes
For an extra treat, top with whipped cream or additional peanut butter.