In a microwave-safe mug, combine the flour, sugar, cocoa powder, baking powder, and salt. Stir well until all dry ingredients are evenly mixed.
Add the milk, peanut butter, vegetable oil, and vanilla extract to the dry ingredients. Mix until you have a smooth batter, making sure to scrape the bottom and sides of the mug.
If desired, fold in a small handful of chocolate chips for extra chocolatey goodness.
Microwave the mug cake on high for 1 minute. Keep an eye on it, as microwave times may vary. If it’s not done, continue to microwave in 10-second intervals until it has risen and is set in the center.
Allow the cake to cool for a minute before enjoying directly from the mug. You can also top it with a dollop of whipped cream or a drizzle of chocolate syrup if desired.
Notes
Microwave times may vary; adjust accordingly. Optional toppings include whipped cream or chocolate syrup.