In a microwave-safe mug, whisk together the flour, sugar, baking powder, and salt until well combined.
Add the milk, peanut butter, vegetable oil, and vanilla extract to the mug. Stir until the batter is smooth and no lumps remain.
Microwave the mug cake on high for 1-2 minutes, or until the cake is set and cooked through. The cooking time may vary depending on the wattage of your microwave, so start with 1 minute and check for doneness.
Let the mug cake cool for a few minutes before serving. Optionally, top with a drizzle of melted peanut butter or a sprinkle of chopped peanuts for extra flavor.
Enjoy your Peanut Butter Mug Cake straight from the mug or transfer it to a plate for a more elegant presentation.