In a 16-ounce microwavable mug or small bowl, stir together flour, sugar, baking powder, and salt.
Using a fork, whisk in milk, melted butter, and vanilla until batter is smooth and there are no lumps.
If mixing in a small bowl, pour into a 12 ounce mug (or larger) to cook. Otherwise, you can cook cake right in the bowl.
Microwave cake for 60-90 seconds, or until cake rises and starts to pull away from the edges of mug and the center is almost firm, but still looks slightly shiny.
Be sure to keep an eye on the cake as it cooks, as it might overflow if your mug isn’t large enough.