In a microwave-safe mug, combine the flour, cocoa powder, sugar, baking powder, and salt. Stir until well mixed.
Add in the milk, peanut butter, vegetable oil, and vanilla extract. Mix thoroughly until you have a smooth batter. If you like, fold in a few chocolate chips for a richer flavor.
Microwave the mug on high for about 1 minute and 20 seconds. Cooking times may vary by microwave, so keep an eye on it. The mug cake is done when it’s puffed up and a toothpick inserted in the center comes out clean.
Let the mug cake cool for a minute or two. You can enjoy it straight from the mug or transfer it to a plate if desired.
Notes
For a delightful touch, consider topping the mug cake with a dollop of whipped cream or a scoop of ice cream and a sprinkle of chopped peanuts for added texture.