Place the butter into your mug and microwave about 30 - 40 seconds or until just melted.
Add remaining ingredients - flour, baking powder, brown sugar, vanilla extract (or substitution), milk, and chocolate chips (if desired). Mix well with a fork.
Microwave 60 - 90 seconds. Start with 60 seconds and then check. You'll know it's done when the cake starts to pull away from the sides of the mug and the top is dry.