In a small bowl, whisk together flour, sugar, baking powder, and cinnamon until thoroughly, completely combined, with no streaks of any ingredients remaining. Blend in milk, butter, and vanilla until batter is smooth. Into a 14-ounce (or larger) microwave-safe mug with straight sides, scoop a big spoonful of batter, then sprinkle with a spoonful of cinnamon sugar. Alternate layers, ending with cinnamon sugar.
Microwave on high for 1 to 1 1/2 minutes, or until cake is done to your liking. Allow to cool for a couple of minutes before serving.
A tall, cylindrical mug with straight (not angled) sides will ensure that the cake bakes evenly. I found a 14-ounce mug to be the perfect size to allow the cake to rise up while baking without overflowing. (I filled my mug to the top with water and then transferred the water to a measuring cup to determine its capacity.) If you use a smaller mug, you may want to bake the cake on top of a plate or paper towel to catch any possible overflow.
The strength of individual microwaves can vary. The first time you make this recipe, be prepared to experiment a bit to make sure that 1 minute is the perfect length of baking time. The nice thing about this cake is that even if it is undercooked, you don't have to worry about raw eggs!
Unsalted butter will work just fine in this recipe, but I find that using salted butter adds a tiny flavor boost.