Creating this single-serving masterpiece is incredibly simple. Follow these steps carefully for a perfect brownie every time. The key is to not overmix and, most importantly, not to overcook.

1. Choose Your Mug: Select a standard-sized, microwave-safe mug (around 12-16 ounces). Using a mug that is too small will cause the batter to overflow, creating a mess in your microwave. A wider, shorter mug is often better than a tall, skinny one as it helps the brownie cook more evenly.
2. Combine Dry Ingredients: In your chosen mug, add the all-purpose flour, granulated sugar, unsweetened cocoa powder, and the pinch of salt. Use a small whisk or a fork to mix these dry ingredients together thoroughly. This step is crucial to ensure there are no lumps of flour or cocoa in the final product.
3. Add Wet Ingredients: Pour the milk, vegetable oil, and vanilla extract directly into the mug with the dry ingredients. Whisk everything together until the batter is just combined. Scrape the bottom and sides of the mug to make sure all the dry flour is incorporated. Be careful not to overmix; stop as soon as you no longer see dry streaks of flour. Overmixing can develop the gluten in the flour, resulting in a tough, rubbery brownie.
4. Add the Peanut Butter: Drop the heaping tablespoon of creamy peanut butter right into the center of the batter. Use a fork or a toothpick to gently swirl the peanut butter into the brownie batter a few times. Don’t fully mix it in! The goal is to create distinct ribbons and pockets of melted peanut butter throughout the brownie for the best flavor and texture experience.
5. Microwave to Perfection: Place the mug in the center of your microwave. Cook on high power. The cooking time is the most critical and variable step, as it depends entirely on the wattage of your microwave.
   • For a 1000-1200W microwave: Start with 60 to 75 seconds.
   • For a 700-900W microwave: Start with 90 seconds to 2 minutes.
     The brownie is done when the top looks set and dry, but a toothpick inserted into the center still comes out with some moist, fudgy crumbs attached (not wet batter). It’s always better to undercook it slightly than to overcook it. An overcooked microwave brownie will be dry and tough.
6. Rest and Cool: Carefully remove the mug from the microwave—it will be very hot! Let the brownie rest on the counter for at least 1-2 minutes. This is a non-negotiable step. The residual heat will continue to cook the center to fudgy perfection, and it allows the molten-hot brownie to cool down to an edible temperature.
7. Enjoy: Grab a spoon and dive right in! Enjoy your warm, gooey, chocolate-peanut butter creation straight from the mug.