In a mixing bowl, combine the flour, sugar, baking powder, and salt.
Add the peanut butter, milk, and vanilla extract to the dry ingredients. Mix well until the batter is smooth and well combined.
Divide the batter evenly into 4 microwave-safe mugs.
Microwave each mug separately on high for 1.5 to 2 minutes, or until the cake is cooked through. Be careful not to overcook.
Let the mug cakes cool for a minute before serving. You can top them with a dollop of whipped cream, chocolate chips, or a drizzle of peanut butter for extra deliciousness.
Enjoy!