Preheat oven to 350°F. In an oven-safe mug (it won't rise too high, so an 8-oz is fine), add all four ingredients. Mix with a mini whisk vigorously until batter is completely mixed and smooth.
Place mug on a baking sheet and then place into oven. Bake for approximately 16-18 minutes (mine took exactly 17 1/2 minutes). Watch the cake carefully towards the end. You want the cake to have puffed up except the center, which should still look dark and not completely set (but no longer wet, like photo above). Let cake cool a few moments before eating. You can top with powdered sugar and eat with ice cream if desired.
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.