In an 8 oz microwave-safe mug, add cocoa, flour and sugar. Whisk together with a mini whisk.
Add in oil and water. Whisk briskly until batter is smooth, making sure to break up any cocoa or flour lumps.
Add half the chocolate to the mug and briefly stir into batter. Sprinkle remaining chocolate on top of the batter.
Cook cake for about 40 seconds at full power. (I used a 1000 watt microwave. You may need to adjust time and/or power if you are using a different wattage.) Top of brownie should no longer be wet but may still be a little moist. Allow brownie to cool a few minutes and then eat the brownie while warm.
Notes
Because the brownie is so rich and fudgy, the single serving size will be small-- just like your typical brownie slice. This recipe will only rise about 2/3 of the way in an 8 oz mug.
It was really hard to take a photo and show the brownie since it sits so low in the mug, so for photo purposes I doubled the recipe. I don't recommend doubling the recipe when you make it though because it cooks more evenly as a single serving. If you plan on making more than one serving, it is better to make two individual brownies rather than a giant one.
The crumb is looser than your typical brownie and you won't be able to take the brownie out of the mug in one piece. It really needs to be eaten with a spoon in the mug.
The reason the crumb is so loose is because I didn't use any egg in this recipe, which normally would act as a binding agent. I think it is too much of a hassle to try to use a fourth of an egg to make this mug brownie, so it's better to eliminate the egg altogether. If the loose texture really bothers you, you can can substitute 1 tbsp of water with 1 tbsp of whisked egg.
This mug cake is best eaten warm a few minutes after it is cooked. The fudgy texture of the warm brownie helps hold the crumbs together. Once it cools, the brownie loses some of its fudginess and the crumbs become looser.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.