I'm a susbstitute-happy cook. There are few recipes I follow to the letter, and I would much rather make do than go to the store for an ingredient I don't have. Also with a dairy-allergic husbeast, it's kind of a must, or I would never find any recipes I like. So this recipe is more of a guideline than anything.
1 1/2 tbsp flour
1 tbsp cocoa, or baker's chocolate, or even chocolate chips in a pinch (just use less sugar)
2 tbsp sugar, or 1tbsp honey, corn syrup, a pack of splenda, whatevs
1 tbsp mayo, or miracle whip, or veganaise, or 1 egg white
1 1/2 tbsp milk, almond/soy/coconut milk, cold coffee for a mocha brownie, or even just straight water.
pinch of salt (optional)
1/4 tsp vanilla (optional but yum, I might even try hazelnut or peppermint extract at some point)
Chocolate chips, nuts, peanut butter, candies, anything you think would be good.
Mix it all up in a mug and microwave for 2-3 minutes. Unless you use egg white, none of these ingredients are a health risk if you under-cook them, so go ahead and make it as fudgy as you like.
Bonus - 130 calories if you use light mayo, skim milk, and no extras. Less than that if you use splenda. But that's no fun ;)