It takes just a few minutes to have this hot, gooey hot fudge mug cake in your hands and ready to scarf down. With a scoop of ice cream on top, you've got a pretty killer dessert that took no time at all!
Place butter in mug and melt in the microwave-- about 15 seconds
Stir 1 tablespoon sugar into the butter followed by the vanilla, flour, 1½ teaspoon of the cocoa powder and milk. Stir until no dry spots remain and smooth into the bottom of the mug.
Sprinkle remaining 1 tablespoon of sugar, ½ teaspoon cocoa powder, and brown sugar over the batter. Pour hot water (hot tap water is fine) over the top of the contents in the mug.
Set microwave to cook at 50% power and cook for 1½ minutes, checking every 30 seconds to see how it's cooking. All microwaves cook differently so keep a close eye on it. It's done when the middle looks cakey. The "hot fudge' portion of the cake will be both on the top and bottom of the mug and will have the consistency of pudding when done. Serve hot!