Whisk together all of your dry ingredients in your mug. Make sure there are no lumps.
Pour in the wet ingredients and whisk, making sure there are no flour pockets left and the batter is smooth. You don't want the batter to take up over half of the mug, otherwise it will overflow in the microwave.
Cook in your microwave for 1-2 minutes, might took about a minute and a half to cook on high but please note that microwaves vary on the wattage.
Let cool for a couple of minutes then eat! I topped mine with whipped cream and more chopped walnuts.
Notes
Storage: I don't recommend storing the mug cake, it is best eaten soon after cooking.Cooking Time: the cooking time depends on the wattage of your microwave, it can take 1-2 minutes. The higher the wattage, the faster it will cook.Mug Size: Don't use a mug that is too small or it will overflow! Ideally the batter should take up about half of the mug before cooking. Additions: Add sprinkles, whipped cream, or ice cream to your mug cake!