In a large microwave-safe mug, combine the flour, sugar, cocoa powder, baking powder, and salt. Mix well to ensure ingredients are evenly combined.
Add the milk, vegetable oil, and vanilla extract to the dry ingredients in the mug. Stir until the mixture is smooth and there are no lumps.
Gently drop the peanut butter into the center of the brownie batter. Use a toothpick or small spatula to swirl it slightly into the batter, but do not mix completely. This will create a delicious peanut butter center.
If you’re using chocolate chips, sprinkle a few on top of the batter for added chocolate goodness.
Microwave the mug on high for about 40-60 seconds. Start checking at 40 seconds. The brownie should look set but slightly moist in the middle; it will continue cooking as it cools.
Allow the brownie to cool for a minute or two before digging in.
Notes
Enjoy straight from the mug or transfer to a small plate. Top with additional peanut butter drizzle or a scoop of vanilla ice cream for extra indulgence.