You would think that the mug simply saying it is microwave safe is enough, but it isn't. After much trial and error, I have found that if your mug gets very hot in the microwave, then it isn't microwave proof. This means that the heat isn’t going to the brownie, it is going to the mug, and it thus won't cook. A mug is really microwave proof if the mug itself stays relatively cool after cooking in the microwave, but the brownie batter gets hot enough to cook and turn into a brownie. I usually use a ramekin for my microwave recipes because they tend to be microwave proof and work best.