Sandy, once I figured out what I wasn't doing right, this recipe came out fantastic!

The first three tries I made in different sizes of coffee cups. I should have realized that a coffee "mug" was not necessarily a coffee cup.

When I finally realized that was my problem,

I got one of my Campbell soup mugs and made my delicious chocolate cake in a mug. It came out perfect!
I opted not to use the chocolate chips. For a splash of vanilla, I put in 1/4 tsp. of vanilla. It was a perfect amount for me. I wasn't sure if I should put the egg in without beating it, so I beat it with a fork in a separate bowl and then poured it in the mug.
I like the cake when it's warm the best. By the time it was cooled off, it was all in my stomach.

I think some icing on top would give it a little pizzazz and I will try that the next time.
This recipe is awesome

and its perfect for someone like me living alone and wanting something sweet. It's not too much and none will be wasted; I won't gain too much weight because I'm not eating a whole cake by myself! It is perfect for taking to work to eat.
Sandy, I agree. This recipe is a
KEEPER and I will be making it alot, especially when I have a craving for chocolate (my biggest weakness. LOL) Thanks for sharing this recipe.
A suggestion I have is to make a note at the end of the recipe to clarify if the mug is a coffee cup or other type of mug. I have friends that want the recipe and I want to be able to tell them the correct type of mug to use.