In a microwave-safe mug, combine the flour, sugar, cocoa powder, baking powder, and salt. Stir well to ensure there are no lumps.
Add the milk, peanut butter, vegetable oil, and vanilla extract to the dry ingredients in the mug. Mix thoroughly until you achieve a smooth batter, ensuring no dry flour remains.
For an extra chocolatey treat, fold in a few chocolate chips into the batter.
Microwave the mug on high for about 40 to 60 seconds. Keep a close eye on it; cooking times may vary depending on your microwave's wattage. The cake should rise and set but still look slightly moist in the center.
Allow the mug cake to cool for a minute before digging in. You can top it with a dollop of whipped cream or a drizzle of melted chocolate for added flavor if desired.
Notes
For extra indulgence, top with whipped cream or melted chocolate.