I know that you are thinking this is a pretty sad cake this week... but I have kids -- and maybe you have kids. If you do have kids they will love it. This cake can be made entirely by a five year old and that's pretty awesome in our house.
The recipe called for a tablespoon of peanut butter to start. I'd switch to 1 1/2 tsp. peanut butter added to the bottom of the mug.
Then whisk together:
1/2 cup Reddi Whip
1/4 cup EggBeaters
1/2 cup dry brownie mix
Pour batter of the peanut butter and microwave for 1.5 minutes.
The mug though, is what will make the difference in a good lookin' cake and this: