Sprinkle on the chocolate chips (if using), and microwave for another 30 to 120 seconds, until the batter is top center is almost dry. The texture will be fudgey, so don't over-microwave because ...

Ensure the mug is 100 per cent microwave safe – often it will say so on the bottom. Check that there are no metallic details in the pattern, which could cause sparks or even a fire.