Oh my gosh, I had heard of similar recipes before (cake in a mug, etc.), but never actually tried one. Then I came across this recipe a few weeks back, and have had it several times a week since then. It's completely addicting. I don't use the coffee (because I hate coffee), and I replace unsweetened cocoa powder with dark cocoa powder. I've added milk chocolate chips, peanut butter chips, peanut butter, nutella, etc. All were good.
But I just found one add in that was GREAT. I'm baking s'more cookies for Father's Day, but that means that we have leftover graham crackers and mini marshmallows. I replaced a little bit of the flour in my microwave brownie with crushed graham crackers and added marshmallows. *swoon*